Add named validation errors

[annevk]

Closes #406.

@TRowbotham I took some time to rebase #502 (what helps a lot is squashing first) and I think it worked out, but I didn't want to overwrite your branch without checking with you.

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[TRowbotham]

Thanks for cleaning this up @annevk! Overwriting my branch is fine.

The only validation error that is missing a description and example is unexpected-windows-drive-letter-host. It is part of file host state step 1.1. I seem to be having a mental block on that one, so if anyone has an idea for that it would be great.

It would be good if someone did a sanity check on the error descriptions and examples too.

[rmisev]

The only validation error that is missing a description and example is unexpected-windows-drive-letter-host. It is part of file host state step 1.1. I seem to be having a mental block on that one, so if anyone has an idea for that it would be great.

Description:
The file URL's host is a windows drive letter.

Example:
"file://c:"

[annevk]

Let's continue this in #502 now it's been rebased.