Expose errors returned by validateFn

[puneetdixit200]

Fixes Or Enhances

Fixes #1575.

validateFn methods that return a non-nil error now keep that error attached to the generated field validation error. Callers can inspect it with errors.Is or errors.As through either the returned ValidationErrors value or the individual FieldError.

This keeps the existing FieldError interface and validation failure shape unchanged while making the underlying Validate() error result accessible.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

Verification

• GOMODCACHE=/tmp/validator-gomodcache GOCACHE=/tmp/validator-gocache go test ./... -run TestValidateFn -count=1
• GOMODCACHE=/tmp/validator-gomodcache GOCACHE=/tmp/validator-gocache go test -cover -race ./...
• PATH=/tmp/validator-bin:$PATH GOMODCACHE=/tmp/validator-gomodcache GOCACHE=/tmp/validator-gocache golangci-lint run
• git diff --check

@go-playground/validator-maintainers

[zemzale]

Seems good.