[BOLT][Wrapper] Don't compare output upon error exit code

Fix llvm-bolt-wrapper to skip output file checks if llvm-bolt exits with error
code.

Test Plan:
- checkout to revision with invalid NFC mismatch in `is-strip.s` test
  (e.g. 056af487~)
- run `nfc-check-setup.py`
- run `bin/llvm-lit -a tools/bolt/test/X86/is-strip.s`

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D143614