jsoncom: gracefully report EMSGSIZE errors

When `jsoncomm` fails because the message is too big it currently
does not indicate just how big the message was. This commit adds
this information so that it's easier for us to determine what to
do about it.

We could also include a pointer to `/proc/sys/net/core/wmem_defaults`
but it seems we want to not require fiddling with that so let's
not do it for now.

See also https://github.com/osbuild/osbuild/pull/1838

osbuild/util/jsoncomm.py

@@ -404,7 +404,13 @@ class Socket(contextlib.AbstractContextManager):
         if fds:
             cmsg.append((socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds)))
 
-        n = self._socket.sendmsg([serialized], cmsg, 0)
+        try:
+            n = self._socket.sendmsg([serialized], cmsg, 0)
+        except OSError as exc:
+            if exc.errno == errno.EMSGSIZE:
+                raise BufferError(
+                    f"jsoncomm message size {len(serialized)} is too big") from exc
+            raise exc
         assert n == len(serialized)
 
     def send_and_recv(self, payload: object, *, fds: Optional[list] = None):