oak-jcr: improve error message when mongo test fixture fails

javax.jcr.RepositoryException: Failed to get Repository instance.
	at org.apache.jackrabbit.test.RepositoryHelper.getRepository(RepositoryHelper.java:72)
	at org.apache.jackrabbit.test.RepositoryHelper.getProperty(RepositoryHelper.java:152)
	at org.apache.jackrabbit.test.AbstractJCRTest.getProperty(AbstractJCRTest.java:526)
	at org.apache.jackrabbit.test.AbstractJCRTest.setUp(AbstractJCRTest.java:318)
	at org.apache.jackrabbit.oak.jcr.JackrabbitNodeTest.setUp(JackrabbitNodeTest.java:56)
	at junit.framework.TestCase.runBare(TestCase.java:140)
	at junit.framework.TestResult$1.protect(TestResult.java:122)
	at junit.framework.TestResult.runProtected(TestResult.java:142)
	at junit.framework.TestResult.run(TestResult.java:125)
	at org.apache.jackrabbit.test.JCRTestResult.run(JCRTestResult.java:75)
	at junit.framework.TestCase.run(TestCase.java:130)
	at org.apache.jackrabbit.test.AbstractJCRTest.run(AbstractJCRTest.java:476)
	at junit.framework.TestSuite.runTest(TestSuite.java:241)
	at junit.framework.TestSuite.run(TestSuite.java:236)
	at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:90)
	at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:365)
	at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:273)
	at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:238)
	at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:159)
	at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:384)
	at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:345)
	at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:126)
	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:418)
Caused by: org.apache.jackrabbit.test.RepositoryStubException: java.lang.reflect.InvocationTargetException
	at org.apache.jackrabbit.test.RepositoryStub.getInstance(RepositoryStub.java:227)
	at org.apache.jackrabbit.test.RepositoryHelper.getRepository(RepositoryHelper.java:68)
	... 22 more
Caused by: java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:490)
	at org.apache.jackrabbit.test.RepositoryStub.getInstance(RepositoryStub.java:225)
	... 23 more
Caused by: javax.jcr.RepositoryException: java.lang.NullPointerException
	at org.apache.jackrabbit.oak.jcr.OakMongoNSRepositoryStub.<init>(OakMongoNSRepositoryStub.java:65)
	at org.apache.jackrabbit.oak.jcr.OakRepositoryStub.newDelegate(OakRepositoryStub.java:74)
	at org.apache.jackrabbit.oak.jcr.OakRepositoryStub.<init>(OakRepositoryStub.java:45)
	... 28 more
Caused by: java.lang.NullPointerException
	at org.apache.jackrabbit.oak.jcr.OakMongoNSRepositoryStub.<init>(OakMongoNSRepositoryStub.java:58)
	... 30 more