More user-friendly error messages

[tfuxu]

Currently when an error occurs while accessing/rendering content, the backend usually returns an error message from Gio or other underlying library, the contents of which can be cryptic for users.

An example would be Gio error: "Could not connect to geminispace.info: Socket I/O timed out" which simply means that server is unreachable.

In addition to adding user-friendly messages, there should also be more error pages, for eg. wrong certificate or unreachable server errors.

Geopard	Epiphany