cpack: Better error message when generator doesn't exist.

Currently the CPack error message when passed an invalid generator
name  reads like the generator failed to work, rather than the
name was invalid.

Source/CPack/cpack.cxx

@@ -358,8 +358,9 @@ int main(int argc, char const* const* argv)
             cpackGenerator->SetTraceExpand(traceExpand);
           } else {
             cmCPack_Log(&log, cmCPackLog::LOG_ERROR,
-                        "Cannot initialize CPack generator: " << gen
-                                                              << std::endl);
+                        "Could not create CPack generator: " << gen
+                                                             << std::endl);
+
             parsed = 0;
           }
 

Tests/RunCMake/CPackCommandLine/NotAGenerator-stderr.txt

@@ -1 +1 @@
-^CPack Error: Cannot initialize CPack generator: NotAGenerator
+^CPack Error: Could not create CPack generator: NotAGenerator