Improve "Could not find type named" error by citing plugin name or index and location in gql schema

[hcharley]

This error:

((DEV)) @my-ns/server: A serious error occurred when building the initial schema (attempt 9). We'll try again in 8100ms. Error details:
((DEV)) @my-ns/server: Error: Could not find type named 'boolean'.

Would better read as:

Could not find type named 'boolean' in plugin 2 'MyCustomPlugin' in 'Query.myQuery'

As discussed just now in Discord.