[lib] Factor out CANCELLED_ERROR into keyserver-conn-types.js

Summary:
In a following diff, I want to use this constant from lib/keyserver-conn/recovery-utils.js. Since that file already imports from keyserver-connection-handler.js, this has the risk of introducing an import cycle. To preempt that I decided to extract the constant to a types file.

I also snuck in an unrelated minor change: I replaced the export at the end of keyserver-conn-types.js with individual export lines at the declarations, to make the style consistent throughout the file.

Depends on D11220

Test Plan: Flow

Reviewers: inka, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11221