Add better error reporting when a plugin fails to load

[jmetz]

In js client, when a plugin fails to load all that's shown in the console window is eg :

Plugin Plugin-Name failed to load in 180s.

Suggest adding debug logging of the traceback / issue. Also in this particular instance it's unclear why a plugin would have taken so long to load, so my guess is it actually failed to load but that didn't get propagated back to the pluginManager somehow.