[LIT] Emit timeout error message only if timeout was reached

Summary:
This improves readability of LIT output: previously
error messages gets emitted that say that there was no error:

error: command reached timeout: False

Patch by Alexey Sachkov.

Reviewers: ddunbar, mgorny, modocache

Reviewed By: mgorny

Subscribers: delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64240

llvm-svn: 365895