Improve error message for base class followed by interface

[adamdruppe]

The spec requires base class to be listed first, but if you flip it, the
compiler complains about multiple inheritance instead of just hinting
that you need to reorder your list. This makes the error message a
little more user-friendly.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21883	minor	poor error message when swapping order of base class and interface

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#12490"

[Imperatorn]

Where's teh tests

[adamdruppe]

I kinda regret leaving that variable called baseClassError since now it is actually counting more like numberOfBaseClasses. but meh.

[Geod24]

The spec requires base class to be listed first

Why is that though ?

[adamdruppe]

I don't know, perhaps just copied from Java's extends X implements Y thing which i believe must also be in that order, but it isn't really bad, the base class is fairly special.