Make validate-element errors a bit more clear

[yumaikas]

So, I've been using joy's html/encode to help get Praxis's html rendering off the ground, and the name must be a keyword error had me spinning my wheels for a few hours, because I thought it was talking about the value of the name attribute.

I've added what I hope are clarifying edits to the validate-element error messages, to help save the next person from getting lost down the same hole I did.

[swlkr]

This is such a good improvement! Thanks!

:shipit:

[yumaikas]

I did deliberately avoid adding dynamic info to the error messages, on the odd chance this error surfaces to a user.