Error nil.type/0 is undefined is hard to diagnose

[joshprice]

As reported by @puruzio on Slack. The stacktrace from this error makes it very hard to pin down where the error is occurring in the schema.

Ideally this error should be pre-empted with a useful warning to avoid the problem.

 ** (UndefinedFunctionError) function nil.type/0 is undefined or private                                                                                                                        
    nil.type()                                                                                                                                                                                 
    (elixir) lib/enum.ex:1188: anonymous fn/3 in Enum.map/2                                                                                                                                    
    (stdlib) lists.erl:1263: :lists.foldl/3                                                                                                                                                    
    (elixir) lib/enum.ex:1188: Enum.map/2                                                                                                                                                      
    lib/graphql/type/schema.ex:111: GraphQL.Schema._reduce_arguments/2                                                                                                                         
    lib/graphql/type/schema.ex:76: anonymous fn/2 in GraphQL.Schema.reduce_types/2                                                                                                             
    (stdlib) lists.erl:1263: :lists.foldl/3                                                                                                                                                    
    lib/graphql/type/schema.ex:75: GraphQL.Schema.reduce_types/2                                                                                                                               
    (stdlib) lists.erl:1263: :lists.foldl/3                                                                                                                                                    
    lib/graphql/type/schema.ex:75: GraphQL.Schema.reduce_types/2                                                                                                                               
    lib/graphql/type/schema.ex:49: GraphQL.Schema.do_reduce_types/2                                                                                                                            
    lib/graphql/type/schema.ex:30: GraphQL.Schema.new/1                                                                                                                                        
    lib/graphql/validation/validator.ex:58: GraphQL.Validation.Validator.validate_with_rules/3                                                                                                 
    lib/graphql.ex:103: GraphQL.execute_with_optional_validation/3                                                                                                                             
    lib/graphql_relay.ex:22: GraphQL.Relay.introspection/0                                                                                                                                     
    lib/graphql_relay.ex:17: GraphQL.Relay.generate_schema_json!/0                                                                                                                             
    (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6                                                                                                                                            
    (elixir) lib/code.ex:168: Code.eval_string/3