Remove raise of StopIteration in ttvdb4 generator

[SteveErl]

Tom Peterson's suggested change to replace the "raise StopIteration" lines with "return" inside the generator routine has been implemented in ttvdbv4_api.py. This is the PEP 479-compliant way to end a generator.

In myth4ttvdbv4.py, the buildSingle() routine has been updated to accommodate a possible StopIteration exception when the generator is called. If that exception happens, we will exit with a "No matching episode found" error message. Similarly, buildCollection() has been updated to exit with a "No matching collection found" error message when the provided reference identifier does not correspond to a known TV series.

The tvmaze metadata grabber had similar vague failures.

For a non-existent episode:

tvmaze.py -l en -a US -D 32938 2 35
ERROR: <class 'AttributeError'> : 'NoneType' object has no attribute 'name'

For a non-existent series:

tvmaze.py -l en -a US -C 99999
ERROR: <class 'AttributeError'> : 'NoneType' object has no attribute 'name'

The tvmaze.py script has been updated to switch these to:

tvmaze.py -l en -a US -D 32938 2 35
ERROR: <class 'SystemExit'> : No matching episode found

tvmaze.py -l en -a US -C 99999
ERROR: <class 'SystemExit'> : No matching collection found

This provides consistent behavior between ttvdb4.py and tvmaze.py grabbers for these types of failures.

Resolves #1433

Checklist

• contribution does not duplicate one of our existing pull requests
• contribution is in a branch rebased against master
• code compiles successfully without errors
• code follows the MythTV Coding Standards
• documentation added/updated/removed where necessary
• commits are logically organised and have good commit messages

[SteveErl]

@rcrdnalor Could you please review #1465
#1466, and #1472? Thanks.