Refactoring of DonorsChoose controller

#### What's this PR do?
In issue #267, our fearless leader @jonuy flagged a few errors logged by Winston. Though the errors were logged, they didn't provide too much information which would allow us to successfully debug. 

In this PR, we've done a few things: 

1) We've added more illustrative information to be logged with errors in Winston. Specifically, mobile phone numbers and full response bodies from the DonorsChoose API. 

2) We've pulled out the the `selectedProposal` parse logic from the try/catch block. Now that our code doesn't need to filter for a certain `costToComplete` property (since our API specifies this in the call), putting this code within the try/catch block is unnecessary. 

3) We've shortened line lengths. Many aren't quite under 80 chars, but we're getting there. 

4) separated out the `donorsChooseApiBaseUrl` into two more variables, `donorsChooseProposalsQueryBaseURL` and `donorsChooseDonationBaseURL` for vlarity. 

#### Where should the reviewer start?
Start with the logic pulled out of the try/catch block (though nothing was changed in the logic.) Then check out the changes to the `logger.error` calls. 

#### How should this be manually tested?
Tested with postman, going through the donations flow. Created a few errors by removing variables and seeing what info was thrown. 

#### What are the relevant tickets?
Related to #263 and #267, but probably doesn't close them.