[JENKINS-57111] Do not put agent offline for exceptions in onOnline

Agent implementers use onOnline to perform actions after an agent
put online.  The API specifies that execeptions thrown by this
method will not put an agent offline.  However, the default
implementation does exactly that.

This change clarifies the API to say all exceptions will be logged
and hardens setChannel against exceptions during onOnline.

NOTE: Errors will still cause an agent to go offline as they are
generally fatal conditions.

Also added tests to verify the behavior.