[lldb][test] Improve invalid compiler error message

I was debugging space separation issues when passing user arguments
and noticed this error is really hard to read in that scenario.

Put "" around the invalid compiler name so you can tell whether
you have spaces around it that's causing the problem.

Author: DavidSpickett

lldb/packages/Python/lldbsuite/test/dotest.py

@@ -248,7 +248,7 @@ def parseOptionsAndInitTestdirs():
             configuration.compiler = which(args.compiler)
         if not is_exe(configuration.compiler):
             logging.error(
-                "%s is not a valid compiler executable; aborting...", args.compiler
+                '"%s" is not a valid compiler executable; aborting...', args.compiler
             )
             sys.exit(-1)
     else: