The --help string states:

  --print-errors                          show verbose information about missing or conflicting packages,default if --cflags or --libs given on the command line

while the code actually lists:

  if (want_my_version ||
      want_version ||
      want_libs ||
      want_cflags ||
      want_l_libs ||
      want_L_libs ||
      want_other_libs ||
      want_I_cflags ||
      want_other_cflags ||
      want_list ||
      want_variable_list)

So I presume that the verbose errors are enabled by default by commands which actually cause output. If that's the intent, the conditional misses some of the options:

- want_provides, want_requires, want_requires_private,
- want_variable.