Add ability to send a general error in all responses from FDI APIs

[rishabhpoddar]

One of the FDI rules is that we should be able to always return a generic error message to the frontend. That's true for some APIs, and not true for others:

• Third party signinup returns a FIELD_ERROR. It would be better to rename it to GENERAL_ERROR
• Email password's sign in / sign up frontend UI can handle a GENERAL_ERROR, but the API doesn't have a way to send that..
• Make sure that all the UI is able to render custom messages sent via GENERAL_ERROR.
• Check for all APIs (except for session refresh) in the FDI to be able to do that or not.
• Change email / sms sending errors to throw errors instead of returning general errors
• In Golang, change onGeneralError function name.
• New FDI version and FDI spec