clang-format-diff.py: pass through errors to stderr, not stdout

Also use write() for unified diff output to avoid further processing by the
print function (e.g. trailing newline).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196484 91177308-0d34-0410-b5e6-96231b3b80d8