fetch: combine unpack error validation in a single function

[ianic]

Follow up of: #19500
discussion

[andrewrk]

I like it this way. What do you think?

[ianic]

I was worried that this one will be harder to test but that was not the problem at the end.
Now all validate is covered. In that sense I like it more.

I also found a bug in previous implementation. Here I'm iterating over all errors but should use only non filtered something like self.errors[0..self.errors_count].