fix : returning error instead of *gqlError.Error

[schafle]

Related Issue:

#99

Background

gqlparser functions are returning nil but the return type is *gqlrrror.Error.

This is how the code looks like before my change: https://github.com/vektah/gqlparser/blob/master/parser/schema.go#L9

func ParseSchema(source *Source) (*SchemaDocument, *gqlerror.Error) {
   .
   .
   .
	return ast, nil
}

The problem is while using this code, err != nil doesn't work since err is actually not nil it is zero value *gqlerror.Error.

The issue had been raised in the past: #99 and they also had a PR for fix #100 which then was closed by author themself.

[coveralls]

Coverage decreased (-0.02%) to 88.562% when pulling f840275 on schafle:gqlerr_to_error into 6d97050 on vektah:master.

[StevenACoffman]

Thanks! This is a very positive change, but given the change to function signatures, I'm wondering if @a8m @giautm and others have thoughts on how to release this. We are currently at v2.4.8, so I wonder if v2.5.0 might be enough of a signal or if we need to go with v3.0.0?