Bring back --hide-error-context flag

[ddfisher]

Also moves hide_error_context out of the experimental section in options.py.

Fixes #2545.

[gvanrossum]

Hm, I wonder if the config variable isn't better named show_error_context...? (Except for backwards compatibility.)

[ddfisher]

It probably is, TBH...

[gvanrossum]

If you want to do that, there's an example for how to provide b/w compatibility for the config file in main.py -- just search that file for all occurrences of silent_imports.