pipe: don't crash on exceptions

pipe: don't crash on exceptions

This commit cleans up the PipeBackend code so it handles all exceptions
itself. Be it initialization errors (i.e. "file not found", "command not
executable"), or error during runtime ("data not returned in pipe
format"). When these errors occur, we now recycle the coprocess so the
rest of the nameserver keeps running. This behaviour is similar to e.g.
an unavailable database server.

Also, make the error-message for ArgMap::asNum display the value of the
setting.

Closes #2619