Catch errors in the API-proxy methods

[ElDeveloper]

As it stands right now, there are individual try/except blocks spread through some methods in the API proxy module, these methods should instead be decorated so that there's a general try/except block that catches any unexpected error and responds with the appropriate error message.

For more information see this discussion:
https://github.com/biocore/qiita/pull/1716/files/8365dceee043c29c50ea3a58e61887e96b3f99a2#r57650625