[JEP-14a] Clarified error type precedence.

[springcomp]

See jmespath.test/#28.

This PR brings the following changes:

    _Each function signature declares the types of its input parameters. If any type constraints are not met,
    implementations must indicate that an `invalid-type` error occurred. **If a function parameter accepts values
    constrained to a specific subset of a type and those constraints are not met, implementations must report
-   that an `invalid-value` error occurred.**_
+   that an `invalid-value` error occurred. How and when those errors are raised is implementation specific.**_

+   _The [initial version of this JEP](./jep-014-string-functions.md) had a provision stating that_ “How and when those
+   errors are raised is implementation specific”. _This provision has been removed. Implementation must perform
+   type-checking for all function parameters_ before _attempting to evaluate the set of valid values for a given type._

[gibson042]

The requirement of type validation before range validation should not be limited to string functions... I think this deserves its own JEP rather than a replacement of JEP-14 (in which case I don't think it will be necessary to touch JEP-14 at all, because "How and when those errors are raised is implementation specific" can be interpreted within the context of that constraint (e.g., "how" might be "by exception" vs. "in a separate return value" and "when" might be "when parsing" [where possible] vs. "when evaluating").

[sebastien-rosset]

I opened two related PRs:
jmespath/jmespath.site#115
jmespath/jmespath.site#116

I know understand I opened them in the wrong project. It seems principles for error handling should be covered in their own section rather than within the context of functions. Should I open these PRs under https://github.com/jmespath-community/jmespath.spec? Or should I start a separate discussion or JEP?

[springcomp]

I opened two related PRs: [...]. Should I open these PRs under https://github.com/jmespath-community/jmespath.spec? Or should I start a separate discussion or JEP?

Yes please open them in the spec repository here, and, by the way, thank you for your feedback.

I think we can start having discussions around existing features as issues, whereas brainstorming new ideas is better served by discussions.