Revert "[lldbtests] Handle errors instead of crashing."

The commit breaks the case where you specify just a filename to the
compiler. Previously, it would look up the compiler in your path, now it
complains that the compiler is not found. One of the lldb buildbots is
depending on this. It seems like a nice feature to have, as it means
less typing and being able to avoid hard-coding the system compiler path
in the bot config.

This reverts commit r316393.

llvm-svn: 316451