Prior to this commit, the schema locations configured with
`spring.graphql.schema.locations` could fail in several cases:
* one of the provided locations cannot be resolved as a Resource
* none of the provided locations holds an actual schema file
The latter can happen if several classpath locations match a given
location and the wrong one is considered first.
This commit introduces a new `MissingSchemaException` that holds the
locations information. We're adding the relevant FailureAnalyzer to help
developers troubleshoot such configurations issues.
Fixes gh-117
625f0b685b