adds check for $EDITOR for the annotate command

[cash]

If $EDITOR is not defined, running annotate results in:

sh: 1: /tmp/tmp2LcDL9: Permission denied
No annotation entered, exiting.

This commit changes that to a helpful error message.

[bilderbuchi]

This code will most probably fail on Windows as there is no $EDITOR (not your PR, the annotate function as it is).
To make it more robust, on Windows you could use os.startfile to open the file in question with the default application.

[cash]

@bilderbuchi os.startfile won't wait for termination of the process.

[robintw]

Thanks :-)

I'll merge this PR now, as it definitely improves on the current situation, and I'll add @bilderbuchi's note about Windows as a separate issue.