Need better error handling in the parser

[Joe Talbott]

1. Don't use .get() to access wiki entry data.
2. Raise exceptions to the top of the stack and log warnings.

[Impact]

 * The parts origin will often change, and some changes might cause errors.

 * Better error messages will make it easier to fix the problems in the source.

[Test Case]

 * Add an invalid wiki entry.
 * Run snapcraft-parser
 * Make sure that an error is printed, but that it's not fatal and the rest of the entries are properly parsed.

[Regression Potential]

 * snapcraft-parse is relatively new. Some other use cases could be affected, so extensive exploratory is required here.

[Leo Arias (elopio)]

https://github.com/ubuntu-core/snapcraft/pull/581