Provide better feedback in UI when input is invalid

[ericglau]

Certain input fields in the UI perform validations on the input, and would show an error on that field if the input is deemed to be invalid.

If a field has an error, no other input takes effect until the error is fixed.
If multiple fields have errors, only the first one is shown. For example, this can be seen when inputting just a in both Voting Delay and Proposal Threshold for Governor.

This can be improved to give better feedback to the user, in case the user did not notice the error. If the user keeps making input changes while there is an error, they may not realize that the resulting contract is not being updated, and may download or copy the stale contract anyways.

Specifically, consider:

• Making input errors more obvious (e.g. always showing the error message, not just when hovering over the field).
• Disabling the Copy, Open in Remix, or Download buttons until all errors are fixed, and show a hover message for those on why they are disabled.
• Allowing multiple errors to be shown at the same time.