Function fz_default_error_callback 

pub unsafe extern "C" fn fz_default_error_callback(
    user: *mut c_void,
    message: *const c_char,
)

FIXME: Better not to expose fz_default_error_callback, and
[fz_default_warning] callback and to allow ‘NULL’ to be used
int [fz_set_xxxx_callback] to mean “defaults”.

FIXME: Do we need/want functions like
fz_error_callback(ctx, message) to allow callers to inject
stuff into the error/warning streams?
/
/**
The default error callback. Declared publicly just so that the
error callback can be set back to this after it has been
overridden.