addon/mg: Trap config::error exceptions thrown during .pbl read

This introduces a new minimal exception object to encapsulate some
relevant information (the file path and the config::error message) so it
can be propagated to the manager UI code correctly.

This is the first step is ensuring faulty .pbl files are not reported
during connection to the add-ons server as a "network communication
error".
addon/mg: Catch and report invalid_pbl_exception exceptions

This allows faulty .pbl files to be reported to the user in a clear
fashion instead of presenting them as a "network communication error"
(which is very wrong, seeing as how these issues only concern local
files).

The actual error is both logged in stderr and presented in the UI as in
the following example:

> Error
>
> A file with add-on publishing information could not be read.
>
> File: /home/shadowm/.wesnoth-1.11/data/add-ons/After_the_Storm.pbl
> Error message: Unexpected characters after variable name (expected , or =), value '' at <unknown>:24