[FacebookBridge] Return error if username starts with slash
[FacebookBridge] Return error if username starts with slash

Requesting a username with a leading slash would cause error 500
because the requested URI would contain two slashes in a row.

For example username "/test" would result in:
https://facebook.com//test

References #628

kranack/rss-bridge,simonbru/rss-bridge,teromene/rss-bridge,sebsauvage/rss-bridge,ORelio/rss-bridge,Riduidel/rss-bridge,kranack/rss-bridge,RSS-Bridge/rss-bridge,RSS-Bridge/rss-bridge,ORelio/rss-bridge,GregThib/rss-bridge,kranack/rss-bridge,teromene/rss-bridge,ORelio/rss-bridge,Riduidel/rss-bridge,Riduidel/rss-bridge,kranack/rss-bridge,Frenzie/rss-bridge,RSS-Bridge/rss-bridge,Frenzie/rss-bridge,simonbru/rss-bridge