Improve error output. Use the third parameter of the "catch" command to

Improve error output. Use the third parameter of the "catch" command to
capture the error output from the exec option. This generally will capture
the stderr messages generated by the tools. This information is then
printed if the test fails. This helps to recognize more quickly what the
error was. Otherwise, this information is lost.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28385 91177308-0d34-0410-b5e6-96231b3b80d8