Fixed <input> logic flaw in _activate_form

This tweaks commit 509a8014dd8d87081927a7c6a53121a3122eb0f0.
Replace Exception by LoginError in _login()
Fixed last failing remote test.
Reference #427 in CHANGES

Fixes #420
Merge pull request #427 from jwoillez/fix_form_activation

Make remote Eso test pass