As of gh-117, the GraphQlAutoConfiguration throws an exception if no
schema file was found while scanning configured locations.
This prevents developers from manually configuring the schema using
`GraphQlSourceBuilderCustomizer` instances.
This commit ensures that a `MissingSchemaException` is only raised if no
schema was configured at the time we're building the actual
GraphQlSource.
This exception is later wrapped by the GraphQlAutoConfiguration to raise
an `InvalidSchemaLocationsException` with useful information about
configured schema locations.
Fixes gh-124
0587913ccb