[errors-] create Error(s)Sheet only if it is not already open

[midichef]

Fixes an inconvenience noted by @daviewales as part of #2573:

if I keep pressing Ctrl+E, duplicate error sheets are created, rather than just jumping me back to the original one.

[midichef]

Should I use an explicit source=None parameter when creating the sheets, in ErrorsSheet("errors_all") and ErrorSheet("errors_recent")? They're getting it initialized like that from BaseSheet already.

[saulpw]

Should I use an explicit source=None parameter when creating the sheets, in ErrorsSheet("errors_all") and ErrorSheet("errors_recent")? They're getting it initialized like that from BaseSheet already.

No, let them use the default.

[anjakefala]

I found a bug with this fix!

To reproduce, put 1/0 in your ~/.visidatarc.
Press ^E, and it will correctly open the ZeroDivisionError.

Then type a column without numerical values with #. Use z^E on one of the cells, and it will incorrectly load the ZeroDivisionError as the cell error.