Select/dropdown - improve error handling and focus state

[mkernohanbc]

This PR contains minor tweaks to the Select component, to address the issues documented in #388 (based on work done for #389):

• 1c0c452: shows a visible error icon to the Select <Button> element when the isInvalid prop is passed
• a56cd53: first pass at implementing validation and error handling using FieldError and ValidationResult. Creates an error message slot underneath the Select <Button>

[mkernohanbc]

Leaving this in draft for the moment, currently having trouble validating whether the errorMessage slot is behaving as expected — need to do some more digging on how to put the component into different error/validation failure states to confirm that this is working.

[mkernohanbc]

Folding some additional tweaks into this PR:

• 45d643e: implements the new focus state style, closes Select - implement new style for focus state #394
• bef7760: fixes spacing between label and input
• f602566 and 332eb56: updates styling of error message and exposes it in the 'Invalid' story

(Took a couple of wrong turns and broke everything along the way, please disregard the commit/revert spam in the middle!)