Style errors more conspicuously and consistently

[waldyrious]

Currently some errors are explicitly marked as errors:

Examples of messages explicitly marked as errors

https://github.com/Zverik/Level0/blob/390c8108c3cd7c7599129542ea48ed6f792213d1/www/osmapi.php#L66

https://github.com/Zverik/Level0/blob/390c8108c3cd7c7599129542ea48ed6f792213d1/www/osmapi.php#L84

https://github.com/Zverik/Level0/blob/390c8108c3cd7c7599129542ea48ed6f792213d1/www/osmapi.php#L90

...and these are styled in red:

https://github.com/Zverik/Level0/blob/390c8108c3cd7c7599129542ea48ed6f792213d1/www/page.php#L13-L15

However, other error messages are merely assigned to $messages:

Examples of error messages that are not marked as such

https://github.com/Zverik/Level0/blob/390c8108c3cd7c7599129542ea48ed6f792213d1/www/osmapi.php#L155

https://github.com/Zverik/Level0/blob/390c8108c3cd7c7599129542ea48ed6f792213d1/www/osmapi.php#L253

...so they appear as regular text in the page rendering, which can mask the fact that there was a problem.

These error messages should also be assigned to $error, or somehow made to be shown in a similar style to make sure the problem is noticed.