cxxmodules: improve error messages for C++ module setup

Make it clear that:

- the quoted string is a target name; and
- C++ sources that export modules is the important detail.

Source/cmGeneratorTarget.cxx

@@ -8927,25 +8927,34 @@ void cmGeneratorTarget::CheckCxxModuleStatus(std::string const& config) const
       case cmGeneratorTarget::Cxx20SupportLevel::MissingCxx:
         this->Makefile->IssueMessage(
           MessageType::FATAL_ERROR,
-          cmStrCat("The \"", this->GetName(),
-                   "\" target has C++ module sources but the \"CXX\" language "
-                   "has not been enabled"));
+          cmStrCat("The target named \"", this->GetName(),
+                   "\" has C++ sources that export modules but the \"CXX\" "
+                   "language has not been enabled"));
         break;
       case cmGeneratorTarget::Cxx20SupportLevel::MissingExperimentalFlag:
         this->Makefile->IssueMessage(
           MessageType::FATAL_ERROR,
-          cmStrCat("The \"", this->GetName(),
-                   "\" target has C++ module sources but its experimental "
-                   "support has not been requested"));
+          cmStrCat("The target named \"", this->GetName(),
+                   "\" has C++ sources that export modules but its "
+                   "experimental support has not been requested"));
         break;
-      case cmGeneratorTarget::Cxx20SupportLevel::NoCxx20:
+      case cmGeneratorTarget::Cxx20SupportLevel::NoCxx20: {
+        cmStandardLevelResolver standardResolver(this->Makefile);
+        auto effStandard =
+          standardResolver.GetEffectiveStandard(this, "CXX", config);
+        if (effStandard.empty()) {
+          effStandard = "; no C++ standard found";
+        } else {
+          effStandard = cmStrCat("; found \"cxx_std_", effStandard, '"');
+        }
         this->Makefile->IssueMessage(
           MessageType::FATAL_ERROR,
           cmStrCat(
-            "The \"", this->GetName(),
-            "\" target has C++ module sources but is not using at least "
-            "\"cxx_std_20\""));
-        break;
+            "The target named \"", this->GetName(),
+            "\" has C++ sources that export modules but does not include "
+            "\"cxx_std_20\" (or newer) among its `target_compile_features`",
+            effStandard));
+      } break;
       case cmGeneratorTarget::Cxx20SupportLevel::Supported:
         // All is well.
         break;

Source/cmGlobalVisualStudio7Generator.cxx

@@ -438,8 +438,8 @@ void cmGlobalVisualStudio7Generator::WriteTargetsToSolution(
     if (target->HaveCxx20ModuleSources() && !this->SupportsCxxModuleDyndep()) {
       root->GetMakefile()->IssueMessage(
         MessageType::FATAL_ERROR,
-        cmStrCat("The \"", target->GetName(),
-                 "\" target contains C++ module sources which are not "
+        cmStrCat("The target named \"", target->GetName(),
+                 "\" contains C++ sources that export modules which is not "
                  "supported by the generator"));
     }
 

Source/cmGlobalXCodeGenerator.cxx

@@ -1374,8 +1374,8 @@ bool cmGlobalXCodeGenerator::CreateXCodeTarget(
   if (gtgt->HaveCxx20ModuleSources()) {
     gtgt->Makefile->IssueMessage(
       MessageType::FATAL_ERROR,
-      cmStrCat("The \"", gtgt->GetName(),
-               "\" target contains C++ module sources which are not "
+      cmStrCat("The target named \"", gtgt->GetName(),
+               "\" contains C++ sources that export modules which is not "
                "supported by the generator"));
   }
 

Source/cmMakefileTargetGenerator.cxx

@@ -201,9 +201,9 @@ void cmMakefileTargetGenerator::WriteTargetBuildRules()
   if (this->GeneratorTarget->HaveCxx20ModuleSources()) {
     this->Makefile->IssueMessage(
       MessageType::FATAL_ERROR,
-      cmStrCat("The \"", this->GeneratorTarget->GetName(),
-               "\" target contains C++ module sources which are not supported "
-               "by the generator"));
+      cmStrCat("The target named \"", this->GeneratorTarget->GetName(),
+               "\" contains C++ sources that export modules which is not "
+               "supported by the generator"));
   }
 
   // -- Write the custom commands for this target

Source/cmVisualStudio10TargetGenerator.cxx

@@ -361,9 +361,9 @@ void cmVisualStudio10TargetGenerator::Generate()
       !this->GlobalGenerator->SupportsCxxModuleDyndep()) {
     this->Makefile->IssueMessage(
       MessageType::FATAL_ERROR,
-      cmStrCat("The \"", this->GeneratorTarget->GetName(),
-               "\" target contains C++ module sources which are not supported "
-               "by the generator"));
+      cmStrCat("The target named \"", this->GeneratorTarget->GetName(),
+               "\" contains C++ sources that export modules which is not "
+               "supported by the generator"));
   }
 
   this->ProjectType = computeProjectType(this->GeneratorTarget);