Convert DecodingErrors thrown from request handling code to HTTP 400 …

[colindignazio]

Motivation

The library does not gracefully handle when clients send requests with malformed request bodies, parameters, etc. If a client sends a request that for example is missing a required field or has a field that cannot be converted to the correct type, the server should respond with a 400 indicating to the client that they have done something wrong. Instead, the library throws a DecodingError which propagates all the way up resulting in a 500 response which incorrectly tells the client that something went wrong on the server.

Modifications

• Added a layer of handling to each call that is made to decode request objects which wraps DecodingErrors into RuntimeErrors.
• Make ServerError conform to HTTPResponseConvertible
• Modify ErrorHandlingMiddleware to first check if the underlying error conforms to HTTPResponseConvertible and if not use the values from ServerError. This allows the HTTPResponseConvertible values set in a RuntimeError to be honoured after the RuntimeError is transformed into a ServerError.

Result

Since RuntimeError conforms to HTTPResponseConvertible these errors will get converted to 400 responses by the ErrorHandlingMiddleware. This isn't a perfect solution because consumers of the library have to opt-in to the ErrorHandlingMiddleware to avoid returning 500.

Test Plan

• Added unit tests for each modification.
• Verified in my own service that with this change malformed requests get converted into 400 responses.

[czechboy0]

Looks great, thank you - I'll let @simonjbeaumont give this one more skim as well before merging.