sideload: bad error message when --arch not given

When you try to sideload a package that is only e.g. on armv7 and you don't specify pmbootstrap sideload --arch=armv7 $pkgname then we write an error message that's not useful at all

sicelo@tpt440p:~/Git/pmos/pmbootstrap$  ./pmbootstrap.py sideload --host 192.168.0.28 linux-postmarketos-omap
[00:16:39] ERROR: Could not find package 'linux-postmarketos-omap'!
[00:16:39] See also: <https://postmarketos.org/troubleshooting>;

Run 'pmbootstrap log' for details.

Before you report this error, ensure that pmbootstrap is up to date.
Find the latest version here: https://git.sr.ht/~postmarketos/pmbootstrap/refs
Your version: 2.2.1

Make sure to at least tell the user they might need to use --arch= to specify the target architecture.