Improve error categorization and cross-platform reporting

[sashei]

In service to sync, the categories of errors (i.e., reasons) needs to be expanded to account for the various condition that can occur. Additionally, the reasons need to be represented in a manner that is understandable to machines and developers across process and platform boundaries.

The additional errors to be handled are covered in sync.

The machine-actionable "reason" is enumerated and reported via a JavaScript Symbol. However, this has some drawbacks:

• Symbols do not have a JSON representation
• Symbols do not have a structured clone representation

The above means a Symbol gets lost whenever the error crosses a process boundary, either as a structured clone or JSON stringification.

For the widest compatibility, the reason property of DataStoreError should be a String instead of a Symbol. Additionally, for improved diagnostic purposes, this reason string should be conveyed in the error message.

Related

• Cloud storage of saved credentials so that they can be accessed from any supported platforms (a single database of credentials for each user) #50

[linuxwolf]

There should be a well-known separator between the error identifier/category and the rest of the message. Is : ok?

LOCKED: the datastore is locked

[linuxwolf]

@mozilla-lockbox/engineering Updated the issue description to reflect the consensus.

[linuxwolf]

mistakenly closed via #71.