1) don't ASSERT; use exceptions (which can be caught) instead.
2) clean up LIB_LOGGER so it's only used during sync
3) fix typos in wxLogMessage() calls so they produce output.
4) throw on unrecognized token in loadContent().
5) if clipboard can't be pasted as content then paste as text.
Fixes: lp:1840172
* https://bugs.launchpad.net/kicad/+bug/1840172