exit on non-EAGAIN failures in multi-reader tests

Ensure we exit our loop when File.Open() fails with something
other than EAGAIN in the CanReadWriteAFileInParallel()
functional test.

Thanks to derrickstolee for catching this on PR review!

GVFS/GVFS.FunctionalTests/Tests/EnlistmentPerFixture/MultithreadedReadWriteTests.cs

@@ -219,6 +219,7 @@ public void CanReadWriteAFileInParallel(FileSystemRunner fileSystem)
                         catch (Exception e)
                         {
                             readException = e;
+                            retry = false;
                         }
                         finally
                         {