Support basic error handling and feedback

[tahini]

• When one of the calls to the trace server returns with an error, the theia frontend should handle it properly, either by doing the appropriate actions to fix it (post again, re-post the trace, etc), or add the error to the status bar at the bottom so the user can see why an output is not shown or what he expected did not happen.

• The frontend should also handle when the trace server is not online

[ebugden]

@bhufmann @mirsky-work Can this be closed?

[bhufmann]

No, error handling still needs to be improved