How to retrieve error details inside the on-error clause

[GeraldSB]

THE CORRECT WAY IS SANITIZE THE STRING FIRST...
for example:

:if ($theurlstring~"^(http|https|ftp|sftp|tftp)://") do={
    # something
} else={
    # do nothing
}

then you can use this example to intercept fetch errors:
fetch capable of following redirects

The question is about getting the on-error detailed info.
The bad URL is just an example that generates an error.