Error if macaroon key is missing from config

Currently we store all access tokens in the DB, and fall back to that
check if we can't validate the macaroon, so our fallback works here, but
for guests, their macaroons don't get persisted, so we don't get to
find them in the database. Each restart, we generate a new ephemeral
key, so guests lose access after each server restart.

I tried to fix up the config stuff to be less insane, but gave up, so
instead I bolt on yet another piece of custom one-off insanity.

Also, add some basic tests for config generation and loading.

synapse/config/__main__.py

@@ -12,6 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from synapse.config._base import ConfigError
 
 if __name__ == "__main__":
     import sys
@@ -21,7 +22,11 @@ if __name__ == "__main__":
 
     if action == "read":
         key = sys.argv[2]
-        config = HomeServerConfig.load_config("", sys.argv[3:])
+        try:
+            config = HomeServerConfig.load_config("", sys.argv[3:])
+        except ConfigError as e:
+            sys.stderr.write("\n" + e.message + "\n")
+            sys.exit(1)
 
         print getattr(config, key)
         sys.exit(0)