* Add crate snafu
* Replace all inline structs in AnkiError
* Derive Snafu on AnkiError
* Use snafu for card type errors
* Use snafu whatever error for InvalidInput
* Use snafu for NotFoundError and improve message
* Use snafu for FileIoError to attach context
Remove IoError.
Add some context-attaching helpers to replace code returning bare
io::Errors.
* Add more context-attaching io helpers
* Add message, context and backtrace to new snafus
* Utilize error context and backtrace on frontend
* Rename LocalizedError -> BackendError.
* Remove DocumentedError.
* Have all backend exceptions inherit BackendError.
* Rename localized(_description) -> message
* Remove accidentally committed experimental trait
* invalid_input_context -> ok_or_invalid
* ensure_valid_input! -> require!
* Always return `Err` from `invalid_input!`
Instead of a Result to unwrap, the macro accepts a source error now.
* new_tempfile_in_parent -> new_tempfile_in_parent_of
* ok_or_not_found -> or_not_found
* ok_or_invalid -> or_invalid
* Add crate convert_case
* Use unqualified lowercase type name
* Remove uses of snafu::ensure
* Allow public construction of InvalidInputErrors (dae)
Needed to port the AnkiDroid changes.
* Make into_protobuf() public (dae)
Also required for AnkiDroid. Not sure why it worked previously - possible
bug in older Rust version?
