Preserving custom scalars validation messages and adding validation errors metadata as extensions (#1686)

[liqweed]

This addresses #1686 .

Custom scalars coercion error messages are preserved in the resulting error.
Also, validation messages have been simplified so they no longer carry concatenated pieces of relevant information to be parsed by clients (arguably making the message less coherent). The relevant information is provided as extensions to the resulting errors.

As an example, instead of:
"Validation error of type WrongType: argument 'arg.foo' with value 'StringValue{value='string'}' is not a valid 'Boolean'"
The validation message would be:
"'string' is not a valid Boolean"
(the original message from the failed scalar coercion) and other aspects of the validation are provided in the error extensions for clients to use directly as values:

"extensions": {
    "argument": "arg.foo",
    "value": "string", 
  	"requiredType": "Boolean", 
  	"objectType": "ArgumentObjectType",
  	"validationErrorType": "WrongType"
}

or in another example, instead of:
"Validation error of type FieldsConflict: scalar: they return differing types Int and String @ 'boxUnion'"
The message would be:
"scalar: they return differing types Int and String"
and the extensions would include:

"extensions": {
    "queryPath": "boxUnion",
  	"validationErrorType": "FieldsConflict"
}

All in accordance with GraphQL errors spec.

Disclaimer: I worked on this before having seen #1725 . I will submit another PR merging the 2 PRs as I see the value of both.

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This change is 

[bbakerman]

Closed as old, not to be merged and in conflict