Make form error messages more descriptive

[vsavkin]

Note: for support questions, please use one of these channels: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question. This repository's issues are reserved for feature requests and bug reports.

• *I'm submitting a ... *
  [ ] bug report
  [x] feature request
  [ ] support request => Please do not submit support request here, see note at the top of this template.
• Do you want to request a feature or report a bug?
• What is the current behavior?

When no value accessor is matched with a standalone element, this is the message we get

ORIGINAL EXCEPTION: No value accessor for ''

• What is the expected behavior?

Provide an error message containing the next action the user has to do to solve the problem.

[kara]

That particular error message was improved in #10051. Other error message improvements in #10286, #10314 :-)

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}