Simplify MintUrl, use it directly in wallet/client.rs

[ok300]

This PR simplifies how MintUrl is handled:

• args that represent a mint URL are given the explicit MintUrl type, instead of Url and a later try_into conversion
• as a consequence, TryInto implementations were not used anymore to convert between MintUrl and Url and were removed
• a few unused error types were also removed (in fact they were duplicated)
• the class method join_url was converted to a method of MintUrl for better readability

[thesimplekid]

Great this is much cleaner. Love a negative git diff