[dwarfdump][NFC] Consistent errors and warnings with --verify

[dwarfdump][NFC] Consistent errors and warnings with --verify

This patch introduces 3 helper functions: error(), warn() and note() to
make printing  during verification more consistent. When supported, the
respective prefixes are printed in color using the same color scheme as
clang.

Differential revision: https://reviews.llvm.org/D38368

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314498 91177308-0d34-0410-b5e6-96231b3b80d8