[dsymutil] Unify error handling and add color

We improved the handling of errors and warnings in dwarfdump's verifier
in rL314498. This patch does the same thing for dsymutil.

Differential revision: https://reviews.llvm.org/D44052

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327137 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit beee6ae8b954383340b48e73029cd8bc722e9554)