Show error message from failed evaluation when doing -var-create

Summary:
When -var-create fails, we will now show the error message from the failed evaluation if it is available.

Patch from chuckr@microsoft.com

Test Plan: I fixed the MiVarTestCase.test_lldbmi_eval test to expect the new error string. All MI tests passing on OS X.

Reviewers: abidh, ChuckR

Subscribers: greggm, lldb-commits, paulmaybee

Differential Revision: http://reviews.llvm.org/D9691

llvm-svn: 237094