dbdump: improve error handling

• Improves error handling during compression of dumps, in order to catch the error code from the mysqldump command. Before, errors in the mysqldump command would fail silently.

• Compression (using gzip) is now an optional flag with --compress.

• The default mysqldump flags can now be overwritten with the CLI argument --params=.

• Re-structures the API a bit from _dump_database to dump_database which takes more arguments.