Problem: mysqlbinlog --raw does not check for fail write errors

Analysis: We have two my_fwrite calls in mysqlbinlog.cc file
which does not check for failures. If my_fwrite returns error and
if we are not catching the error we could end up in corrupting
written out binary logs without reporting any errors/warnings.

Fix: catch my_fwrite return values in those two places
and throw error if fails.