Properly raise an exception when the body exceeds the max size. (#9145)

...instead of just creating the exception object and doing nothing with it.

changelog.d/9145.bugfix

@@ -0,0 +1 @@
+Fix "UnboundLocalError: local variable 'length' referenced before assignment" errors when the response body exceeds the expected size. This bug was introduced in v1.25.0.

synapse/http/client.py

@@ -724,7 +724,7 @@ class SimpleHttpClient:
                 read_body_with_max_size(response, output_stream, max_size)
             )
         except BodyExceededMaxSize:
-            SynapseError(
+            raise SynapseError(
                 502,
                 "Requested file is too large > %r bytes" % (max_size,),
                 Codes.TOO_LARGE,

synapse/http/matrixfederationclient.py

@@ -996,7 +996,7 @@ class MatrixFederationHttpClient:
             logger.warning(
                 "{%s} [%s] %s", request.txn_id, request.destination, msg,
             )
-            SynapseError(502, msg, Codes.TOO_LARGE)
+            raise SynapseError(502, msg, Codes.TOO_LARGE)
         except Exception as e:
             logger.warning(
                 "{%s} [%s] Error reading response: %s",