Calculator error should be more readable, better error parsing should be implemented

[mvadkert]

Description of problem:
If you start calculator activity and hit several times Enter, you get an error:

Error at 0: Parse error

Version-Release number of selected component (if applicable):
0.85.4-1

How reproducible:
always

Steps to Reproduce:

1. start calculator activity
2. hit Enter button several times
3. observe output

Actual results:
Error at 0: Parse error

Expected results:
No error, blank screen.

Additional info:
Also the input filtering could be better, try to enter a very long number and
hit Enter -> the screen gets mixed up. See attached screenshot.

[mvadkert]

mixed up screen after a long input in calculator

[mvadkert]

This bug was found during F12 test day - Sugar on a stick, and reported to RedHat bugzilla ​https://bugzilla.redhat.com/show_bug.cgi?id=521073

[garycmartin]

Testing in Calculate-31 I can't reproduce the 'hit several times Enter, you get an error' issue, and the 'mixed up screen after a long input in calculator' item was an issue during Sugar's the transition from Matchbox to Metacity. It affected several other activities and was fixed in Sugar some time back.