Update NNAS response/error handling

[jonbarrow]

Resolves #160

Changes:

Centralizes the response creation for NNAS. These changes automatically apply the common headers, as well as adds a comment to every response warning users not to share details with untrusted people. Also adds some notes/groundwork for supporting JSON mode in NNAS. Also fixes 3DS cert parsing as a bonus

Some of our error responses were formatted incorrectly, omitting the outer <errors> tag. This resulted in 102-2402 (BAD_FORMAT_REQUEST) since the client could not handle the error response. Now that this is fixed, we should get more accurate error codes from users

Marking as draft since it still needs robust testing on a console. I did some quick sanity checks using curl, but that's not enough

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• What I'm implementing was an approved issue.
• I have tested all of my changes.