Our terminology for errors is slightly wrong, and it doesn't quite
match the model that rust-cssparser and servo expect.

Rust-cssparser provides BasicParseError, which is a
BasicParseErrorKind plus a location.  This is just a simple parsing
error like "unexpected token" or "premature EOF".

On top of that, rust-cssparser provides ParseError and ParseErrorKind.
The former is a ParseErrorKind plus a location; the latter is a basic
parse error, or a custom error.

When implementing the rules_and_declarations traits, cssparser expects
us to provide our own custom parsing errors in the form of
ParseError<OurCustomError>.  For example, Servo has this:

  pub type ParseError<'i> = cssparser::ParseError<'i, StyleParseErrorKind<'i>>;

And that StyleParseErrorKind has a ton of stuff:

  pub enum StyleParseErrorKind<'i> {
      BadUrlInDeclarationValueBlock(CowRcStr<'i>),
      BadStringInDeclarationValueBlock(CowRcStr<'i>),
      UnbalancedCloseParenthesisInDeclarationValueBlock,
      UnbalancedCloseSquareBracketInDeclarationValueBlock,
      UnbalancedCloseCurlyBracketInDeclarationValueBlock,
      PropertyDeclarationValueNotExhausted,
      UnexpectedDimension(CowRcStr<'i>),
      ...
  }

So, we will move to a similar model.

AttributeError is the wrong name; it represents a parsing or
validation error in a value from an element attribute, or a CSS style
property... but we don't know which.  Instead, we'll call it
ValueErrorKind, to make it closer to rust-cssparser's view of the world.