ENH: New format for warning and error messages

- Add cmMakefile methods IssueError and IssueWarning
  - Maintain an explicit call stack in cmMakefile
  - Include context/call-stack info in messages
  - Nested errors now unwind the call stack
  - Use new mechanism for policy warnings and errors
  - Improve policy error message
  - Include cmExecutionStatus pointer in call stack
    so that errors deeper in the C++ stack under
    a command invocation will become errors for the
    command

Source/cmFunctionCommand.cxx

@@ -86,7 +86,7 @@ public:
 
 bool cmFunctionHelperCommand::InvokeInitialPass
 (const std::vector<cmListFileArgument>& args,
- cmExecutionStatus &)
+ cmExecutionStatus & inStatus)
 {
   // Expand the argument list to the function.
   std::vector<std::string> expandedArgs;
@@ -157,18 +157,12 @@ bool cmFunctionHelperCommand::InvokeInitialPass
   for(unsigned int c = 0; c < this->Functions.size(); ++c)
     {
     cmExecutionStatus status;
-    if (!this->Makefile->ExecuteCommand(this->Functions[c],status))
+    if (!this->Makefile->ExecuteCommand(this->Functions[c],status) ||
+        status.GetNestedError())
       {
-      cmOStringStream error;
-      error << "Error in cmake code at\n"
-            << this->Functions[c].FilePath << ":" 
-            << this->Functions[c].Line << ":\n"
-            << "A command failed during the invocation of function \""
-            << this->Args[0].c_str() << "\".";
-      cmSystemTools::Error(error.str().c_str());
-
-      // pop scope on the makefile and return
-      this->Makefile->PopScope();
+      // The error message should have already included the call stack
+      // so we do not need to report an error here.
+      inStatus.SetNestedError(true);
       return false;
       }
     if (status.GetReturnInvoked())