[runtest compile] Improve handling of command failures/unexpected stderr.

 - We now log a more readable diagnostic in situations that error during normal
   invocation of failing commands.

 - We now treat it as a fatal error when we can't parse timing output in other
   situations, which indicates a bug in the testing logic.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@211719 91177308-0d34-0410-b5e6-96231b3b80d8