This checks the avatar size on the client side (if available) and
on the server side against a configuration-defined limit. The
default limit is set to use the same value as in the original
report, as no sensible limit value is known.
Fixes#67.
Previously, some kinds of errors would throw nice and fun cascades
of exceptions.
We now have a nice, clean error page for 500 and 503 (backend
connectivity) errors which includes minimal debugging information
for productive setups and a traceback for development setups.
In any case, the full exception is logged to the log with an error
ID which is printed on the error page.
