Better error messages on empty input and various crashes

[oyvindln]

If you input an empty file you get a unhelpful message like

Traceback (most recent call last):
  File "/home/usrname/vhs-decode/vhsdecode/process.py", line 212, in build_json
    black = jout["videoParameters"]["black16bIre"]
TypeError: 'NoneType' object is not subscriptable
Cannot build json: 'NoneType' object is not subscriptable
saving JSON and exiting
Traceback (most recent call last):
  File "/home/usrname/vhs-decode/vhsdecode/process.py", line 212, in build_json
    black = jout["videoParameters"]["black16bIre"]
TypeError: 'NoneType' object is not subscriptable
Cannot build json: 'NoneType' object is not subscriptable

The same message is often output for other erros with or without other messages which can be confusing, need to add in something a bit more explanatory here.