Fixed typo in logger call.
Fixed incorrect exception handling.
Fixed bug when adding multiple selectors for single channel.

SLNetStation.appendSelectors() expects a string.
Fixed missing argument for SeedLinkException.
Fixed missing check for emtpy value.
Added regression tests for #777.
Changelog for #777
Merge pull request #777 from bmorg/fix_seedlink_bugs

Fixing multiple SeedLink bugs
some flake8 (was fixed in master by 1ca9bfee42e08af7ef57b4e8ad68dc766351bfa3 and 6939a5dc4a57ff37459b37aa954a65e7f3cf3ab5)