FileLfsServlet: Return HTTP 422 instead of 400

According to the specification [1], the error response status code
should be 422 when there is a validation error with one or more of
the objects in the request

[1] https://github.com/github/git-lfs/blob/master/docs/api/v1/http-v1-batch.md#response-errors

Change-Id: Id03fe00a2109b896d9a154228a14a33bce5accc3
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

org.eclipse.jgit.lfs.server.test/tst/org/eclipse/jgit/lfs/server/fs/DownloadTest.java

@@ -78,7 +78,7 @@ public class DownloadTest extends LfsServerTest {
 			getContent(id.name().substring(0, 60), f);
 			fail("expected RuntimeException");
 		} catch (RuntimeException e) {
-			assertEquals("Status: 400 Bad Request",
+			assertEquals("Status: 422 Unprocessable Entity",
 					e.getMessage());
 		}
 	}
@@ -93,7 +93,7 @@ public class DownloadTest extends LfsServerTest {
 			getContent(id.name().replace('f', 'z'), f);
 			fail("expected RuntimeException");
 		} catch (RuntimeException e) {
-			assertEquals("Status: 400 Bad Request",
+			assertEquals("Status: 422 Unprocessable Entity",
 					e.getMessage());
 		}
 	}

org.eclipse.jgit.lfs.server/src/org/eclipse/jgit/lfs/server/fs/FileLfsServlet.java

@@ -121,14 +121,14 @@ public class FileLfsServlet extends HttpServlet {
 			HttpServletResponse rsp) throws IOException {
 		String info = req.getPathInfo();
 		if (info.length() != 1 + Constants.LONG_OBJECT_ID_STRING_LENGTH) {
-			sendError(rsp, HttpStatus.SC_BAD_REQUEST, MessageFormat
+			sendError(rsp, HttpStatus.SC_UNPROCESSABLE_ENTITY, MessageFormat
 					.format(LfsServerText.get().invalidPathInfo, info));
 			return null;
 		}
 		try {
 			return LongObjectId.fromString(info.substring(1, 65));
 		} catch (InvalidLongObjectIdException e) {
-			sendError(rsp, HttpStatus.SC_BAD_REQUEST, e.getMessage());
+			sendError(rsp, HttpStatus.SC_UNPROCESSABLE_ENTITY, e.getMessage());
 			return null;
 		}
 	}