Improved error handling

[sebastien-rosset]

This PR improves error handling.

1. Consistently return SyntaxError. Previously, some syntax errors were reported as SyntaxError and in other cases it was just an error string.
2. Add typ field in SyntaxError. The typ is set to the error string values defined in the JMESpath specification such as invalid-type and unknown-function.
3. Create constants for error types that are defined in the specification.
4. In compliance tests, validate the actual error type matches the expectation. Previously, the compliance suite was only validating the error is not nil.