Extend use of custom result formatters to CLI tool

[lonvia]

The API functions of the nominatim CLI tool can now also show the outputs created by custom result formatters. There is a new option --list-formats for all of them, which can be used to check, which formats are configured. This should particularly come in handy when you are writing your own formatters. In contrast to the server, the CLI tool will properly report on any exceptions in the code with the usual Python backtrace making debugging easy.

Also corrects and improves the documentation on result formatters:

• add section about error message formatting
• add some missing imports in the examples
• add a hint about debugging formatters using the CLI tool
• correct a comment that indicated that new formats must be implemented for all result types, that's simply not true