This change was missing in 47fa8c4c

The CurlRequest constructor can throw exceptions, therefore we must
not use "noexcept".

Allows catchers to inspect the HTTP status.

See commit be137a19

Eliminate duplicate code.

Fixes a freeze bug.

These have no use, exception for creating an artificial dependency
from input_plugins[] to all archive plugins.

These methods are not exception-safe; std::lock_guard should better be
used.

Don't keep any references to the owner, as the handler method may
delete the owner.

The caller should be able to safely store the instance pointer before
his callback is invoked.