DirectUpload returns errors as strings, including an HTTP status and a
file name (and without a stack trace).
But Sentry groups issues according to the stack trace, and maybe the
error message in last resort.
So we have an issue: as all DirectUpload errors logged by Sentry are
generated on the same line, with random-looking messages, Sentry groups
them either too or too little aggressively.
Instead of creating all the errors on the same line:
- add some `if`s statements to create them on different lines (and so
with different stack traces),
- strip the file name from the error message.
This allows Sentry to group the errors properly, with meaningful error
messages.
Under some circumstances (like dispatching events just before a page
navigation), IE 11 events will be dispatched twice by the browser: once
with the payload, and a second time without.
To prevent these errors, ignore the events if the payload is missing.
