Log: libcib: Drop crm_perror() in get_shadow_file()

• Set errno to 0 before calling getpwuid(), as the man page instructs.
• Improve error message, noting that errno might not get set.
• Use correct format specifier for uid_t.
• Drop from error level to warning level.

The only thing we lose here is writing the message to stderr for CLI
tools. However, CLI tools have verbose options. This should be a pretty
rare condition anyway.

Ref T172

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>