Should error when source/backtrace(false) given on wrong field

[tjkirch]

source(false) should only be accepted on fields named "source", and backtrace(false) should only be accepted on fields named "backtrace". Otherwise, the user has likely misinterpreted their purpose.

This is a small follow-up to the misuse errors added in #109 for issue #105.

[tjkirch]

If someone is interested in jumping on this, here's what I'd recommend:

• In snafu-derive/src/lib.rs, scan through fn parse_snafu_enum to get a sense of the error handling, particularly where you see errors.add with a OnlyValidOn
• Where you see the "opt_out" variables being set, we want to check if the field name (name) is what we expect before flipping the boolean
• Add compile-fail tests for each of the attributes that will now fail (look in compatibility-tests/compile-fail/tests/ui)

If no one bites, I'd like to do it before 0.5 since it's related to the other error message improvements and should go in the same breaking-change release :)

[tjkirch]

I've got this ready in tjkirch@70d974b but I can't make a PR until #141 is in -- GitHub won't let me create a PR in this repo against a branch in my fork. Without that ability, a PR would also show all of #141. (The implementation is a fair bit different with #141, so I didn't see a point in implementing pre-#141.)