Improved timeout error handling in feed commands

[ittner]

I was facing some timeout issues with feed-generating commands that may be affecting other users too so follows three commits with small improvements. They are separate changes, but also closely related and mutually dependent, so they all go in a single PR.

First commit allows overriding it via environment variable LIFEREA_FEED_CMD_TIMEOUT, in the same way Liferea already uses LIFEREA_UA for the user-agent string; second increases the default timeout to 60 s (it seems I was too optimistic with the 30 s timeout when I first added it), and the third commit changes the error reporting so it becomes possible to tell timeouts and other errors apart.

I hope they will be useful!

[ittner]

Quick update to the PR, adding another commit with descriptive strings for HTTP server errors (status codes 5xx).

[lwindolf]

PR looks good.