[TestBase.runCmd] Better error message when runCmd fails.

Summary:
Before:
    AssertionError: False is not True : Process is launched successfully

After:
    AssertionError: False is not True : Command 'run a.out' failed.
    >>> error: invalid target, create a target using the 'target create' command
    >>> Process could not be launched successfully

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, vharron

Differential Revision: http://reviews.llvm.org/D9948

llvm-svn: 238363