Guard know code paths in error handling

[mgreter]

Silently drop error json when we cannot generate it due to internal problems.
Improves reported error message by a lot (#2016)

Error: Invalid CSS after "...pecific grammar": expected expression (e.g. 1px, bold), was ". And this thing he"
        on line 1 of foo.scss
>> _((Classes and IDs must follow a specific grammar. And this thing here doesn\x{fffd}t

[coveralls]

Coverage remained the same at 78.766% when pulling 9cb6a20 on mgreter:feature/guard-json-stringify into 445ad3c on sass:master.

[asottile]

Haven't found anything concrete yet, but this commit seems to (somehow?) contribute to a stack overrun: sass/libsass-python#149 (comment)

Hopefully I'll have more information soon :)

[tony]

^ Yup apparently its only on Python 3.5 64bit.

https://ci.appveyor.com/project/dahlia/libsass-python/build/322

edit: 3.5 is the only 3.x 64bit being tested on libsass-python atm