VS10: stop build on custom command error (#11533)

In VS9 and previous versions, :VCReportError is the goto label
to jump to after a failed custom command. It stops the build
before it tries to go any further.

In VS10, :VCEnd is the correct label to use.

Create a method in the VS generators to provide the correct
line of script to use for each version of Visual Studio.

For more internal details, search for VCEnd in the
C:\Program Files\MSBuild directory.

Source/cmLocalVisualStudio10Generator.h

@@ -36,6 +36,10 @@ public:
   virtual void Generate();
   virtual void ReadAndStoreExternalGUID(const char* name,
                                         const char* path);
+
+protected:
+  virtual std::string CheckForErrorLine();
+
 private:
 };
 #endif