Handle errors and warnings in a consistent way

[lognaturel]

This issue is to track the broader approach to error handling.

User stories

As an end user filling out a web form
I want to see clear error messages that are styled consistently with the host app
So that I can understand what happened when something goes wrong

As a host application developer
I want the option to let web forms show their own error messages in a consistent way
So that I don't have to think about error states at all

As a host application developer
I want the option to display error messages in a way that is appropriate for my host app
So that I can create a custom user experience

Types of errors

• Initialization errors (e.g. invalid XML form def)
  • Could always result in a rejected promise
• Bugs we haven't caught yet
• Form design issues
  • Missing secondary instance or missing subpath into it
  • Typo in expression, especially things like second part of if which isn't run at initialization time (but there is/could be static analysis)
  • Unknown functions (maybe we warn?)
  • Divide by 0, other inevitable runtime issues

Types of warnings

• More than 1 node when at most one is expected (label, hint, etc) Initial support for appearances, body classes #127 (comment)

Notes

• Use error condition return values rather than throwing
  • So we can specify what errors are possible
  • For saner control flow (we'll have to make sure to use catch-alls to handle unknown unknowns)
• Create a clear interface for the UI to consume error messages, first for our own internal use, and eventually to let clients consume them in a custom way
• Aim for a dependency-injection type of pattern for the client