Include screenshot size in all error messages

[suve]

Screenshot validation includes checking the image size, i.e:

• whether the width isn't too small
• whether the height isn't too small
• whether the width isn't too large
• whether the height isn't too large

Currently, when the first check fails, the error message includes the actual screenshot width. This is not the case for other checks - they do not contain any information about the dimensions. This patch fixes this inconsistency by making all four checks include the relevant information in their error message.

[hughsie]

Medium term I think we should point people at appstreamcli. Does this PR still pass ninja test?

[suve]

Does this PR still pass ninja test?

[65/66] Running all tests.
1/2 asb-self-test        OK              0.09s
2/2 as-self-test         OK              0.19s

Ok: 2
Expected Fail: 0
Fail: 0
Unexpected Pass: 0
Skipped: 0
Timeout: 0

> Medium term I think we should point people at `appstreamcli`.

[Fedora's packaging guidelines on AppStream files](https://docs.fedoraproject.org/en-US/packaging-guidelines/AppData/) require running `appstream-util validate-relax` on said files. I guess long term we'd want to switch to `appstreamcli` here, as well?

[hughsie]

I guess long term we'd want to switch to appstreamcli here, as well?

I think so. Thanks!