Add play and pause commands

Add commands that always play and always pause, in addition to the
current toggle pause command.

Closes #342.
piano: Add error message for code 1039

See #352.
Stop playback if audio file is not accessible

See #352.
Transform actual station when loving/banning songs

Loving/banning a song from a shared station while playing quickmix
resulted in “call not allowed”, because we tried to transform the
quickmix instead of the song’s real station. Fixes #354.
Fix station name when deleting a station
Add missing events to genre station selection

Two new events: stationfetchgenre, stationaddgenre
Ignore HTTP status 400

Workaround for #355, fixes commit
2c516503d2cb81dd156afc24677ac4bf3caefceb.