[Patch][LNT] fix python error in compile.py when runN failed

[Steven Wu]

LNT currently throws python errors when runN failed in compile.py. It throws error messages about it cannot parse stdout, stderr is not empty and runN does not return 0. 
Ideally, the check after runN should performed in the opposite order. stdout shouldn’t be parsed if runN does not return 0. Since it is expected that runN can fail during the test, g_log.error should be used to print the error instead of throw python error messages.

Please commit for me after review.