In 'fsfsfixer', improve diagnostic messages.

* contrib/server-side/fsfsfixer/fixer/fix-rev.py
  (run_cmd): Print any non-zero return code.
  (replace_in_file): Catch more errors. Don't print the "Replacing ..."
    message unless the replacing succeeds.
  (fix_id, fix_checksum): Don't print the "Fixing ..." message unless the
    fixing succeeds.
  (fix_delta_ref): Raise a more informative error than "Failed to substitute
    ..." if the offset we're supposed to fix appears to be already correct.
    Don't print the "Fixing ..." message unless the fixing succeeds.
  (handle_one_error): Print the error message we're examining, in all cases.
    Print the warning if we don't recognize the error message here rather
    than in the caller, to avoid printing it if in fact we recognized the
    error message but failed to fix the problem.
  (grab_stderr): Log the command being run. Simplify Python code.
  (fix_one_error): Print more accurate messages.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554982 13f79535-47bb-0310-9956-ffa450edef68