VS10: Fix working directory of consecutive custom commands (#11938)

The VS 10 msbuild tool uses a single command shell to invoke all the
custom command scripts in a project.  Isolate the environment and
working directory of custom commands using setlocal/endlocal.  The
form of each command is

  set errlev=
  setlocal
  cd c:\work\dir
  if %errorlevel% neq 0 goto :cmEnd
  c:
  if %errorlevel% neq 0 goto :cmEnd
  command1 ...
  if %errorlevel% neq 0 goto :cmEnd
  ...
  commandN ...
  if %errorlevel% neq 0 goto :cmEnd
  :cmEnd
  endlocal & set errlev=%errorlevel%
  if %errlev% neq 0 goto :VCEnd

so that all changes to the environment and working directory are
isolated within the script and the return code is preserved.

Source/cmLocalVisualStudio10Generator.h

@@ -38,7 +38,7 @@ public:
                                         const char* path);
 
 protected:
-  virtual std::string CheckForErrorLine();
+  virtual const char* ReportErrorLabel() const;
 
 private:
 };