CLI exit codes

• 0 when matches were found and no errors occurred
• 1 when no matches were found in any file
• 64 when query could not be parsed (exitcode::USAGE)
• 66 when no files were selected for search (exitcode::NOINPUT)
• 74 when a file could not be read unless --suppress-read-errors (exitcode::IOERR)
• 74 when a file could not be parsed unless --suppress-parse-errors (exitcode::IOERR)