Explain Serde deserialization issues clearly in error messages

[calavera]

When a function cannot deserialize the incoming data into the type that the handler receives. The runtime returns a Serde deserialization error. We've seen many reports showing that those messages are confusing because they are very sparse. I believe improving the those error messages would remove a lot of the friction that people bump into when using the runtime.

These are some examples of those messages:

• "field_error": "expected value at line 1 column 1",
• data did not match any variant of untagged enum LambdaRequest #599
• Error("missing field identity_id", ...) when invoking with temporary credentials #559
• lambda_http(cargo lambda start): failed to serialize lambda-url event #528

[github-actions]

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