request: add missing early return to complete error check

request: add missing early return to complete error check

While $self->error will immediately send out a 4xx or 5xx response
anyhow its still good to cover against possible side effects (e.g.,
from future code in that branch) on the server and return directly.

Note that this is mostly for completeness sake, we already have
another check that covers this one for relevant cases in commit
580d540ea907ba15f64379c5bb69ecf1a49a875f.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>