DirCacheBuilderTest#testBuildRejectsUnsetFileMode: Add missing fail()

Error Prone reports:

  Not calling fail() when expecting an exception masks bugs

See https://errorprone.info/bugpattern/MissingFail

Change-Id: Ic89f9daef3c31bfbfd1f3c003cb90a373cd74847
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/dircache/DirCacheBuilderTest.java

@@ -89,6 +89,7 @@ public class DirCacheBuilderTest extends RepositoryTestCase {
 		assertEquals(0, e.getRawMode());
 		try {
 			b.add(e);
+			fail("did not reject unset file mode");
 		} catch (IllegalArgumentException err) {
 			assertEquals("FileMode not set for path a", err.getMessage());
 		}