Error messages for deserialization failures and constraint violations

[david-perez]

smithy-rs services have returned HTTP 400 since #1000 (v0.34.0) and an X-Amzn-Errortype header set to SerializationException since #1233 (v0.39.0).

We don't put detailed messages in the HTTP body, e.g. things like invalid character at line 5, character 3: expected START_ARRAY when the input data fails to deserialize, or things like:

{
    "message": "1 validation error detected. Value null at '/string' failed to satisfy constraint: Member must not be null",
    "fieldList": [
        {
            "message": "Value null at '/string' failed to satisfy constraint: Member must not be null",
            "path": "/string"
        }
    ]
}

(example protocol test)

when the input data does not satisfy a constraint.

---

This issue tracks:

• better error messages for deserialization failures; and
• better error messages for constraint violations.

[david-perez]

Constraint violation error messages need implementation of constraint traits (pattern, range, length) as a prerequisite, for which #1342 first needs to land.

[david-perez]

I've ticked off the second item; since 0.52 we now return ValidationExceptions in responses when constraint traits are not honored.

Only pending thing to close this is provide better error messages for deserialization failures things like e.g. invalid character at line 5, character 3: expected START_ARRAY when parsing invalid JSON.