Handle InvocationException correctly

[gurel]

When method.invoke throws an InvocationTargetException the original exception is hidden in the cause of this exception. The SchemaModule then wraps that with another RuntimeException.

Because of these the correct error never reached the GraphQL Execution.

When gRPC service throws:

throw Status.UNIMPLEMENTED
                .withDescription("message from service")
                .asRuntimeException();

The messages below gets rendered on the error.

Before:

Exception while fetching data (/greetingWithException) : java.lang.reflect.InvocationTargetException

After:

Exception while fetching data (/greetingWithException) : UNIMPLEMENTED: message from service

I also added a test case to check whether GraphQLError is also handled correctly.

[coveralls]

Pull Request Test Coverage Report for Build 80

• 4 of 5 (80.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 41.813%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rejoiner/src/main/java/com/google/api/graphql/rejoiner/SchemaModule.java	4	5	80.0%
<!--	Total:	4	5	80.0%	-->

Totals
Change from base Build 79:	0.2%
Covered Lines:	807
Relevant Lines:	1930

---

💛 - Coveralls

[siderakis]

Nice, thanks!