Description of Problem:
When problems occur during scheduled mail checks, it is unhelpful for an
error dialog to pop up (see title "Evolution Error", headline "Error while
Fetching Mail").

For checks that I manually initiate (keyboard "F9" or menu "Send /
Receive"), it is important for me to know if the check failed. However, for
scheduled checks, it's not helpful for me to know that one check failed at
some point in the past (because a more recent successful check would
eliminate the impact of the error).

Suggestions:
* Include a time stamp with the error so I can troubleshoot the failure
* Automatically close the error dialog if a later scheduled check succeeds
* Put a non-dialog error somewhere else in the UI with a message like "Last
mail check failed" / "Last %d mail checks failed".


Steps to reproduce the problem:
1. Set up an account on a server that occasionally refuses connections
2. Enable automatic mail checking
3. Leave the machine for awhile
4. Come back to an error message that gives no indication whether a problem
currently exists.

Actual Results:
I'm unsure whether I have my latest mail since I can't tell when the error
occurred.

Expected Results:
I'm only bothered if an error is _currently_ impacting me.