Fix many issues regarding $errors variable and @ character

[pupi1985]

I've removed many usages of the @ sign and initialized the @errors variable in all pages I noticed it was used.

I have not removed all @ signs, but this should get the code one step closer to that.

This PR should also fix #888.

[svivian]

Thanks for this. We ought to move to PHP 7 minimum soon, so we can write $errors['key'] ?? null instead of all the isset() calls. I've done that in other projects and it makes things a lot neater!

[pupi1985]

I know, I'm looking forward for that to happen :) The PHP 8 release can be used as an excuse for this dependency update.

The good thing of having the wordy isset()s in the code (compared to the @ sign) is that IDE's should be able to detect the "code smell" and hint the fix.