PER-8255: Change error message

[cecilia-donnelly]

We use "connection error" as our default error message, which is
confusing to users. Change the name of the variable to better reflect
the problem and the message to users similarly.

[meisekimiu]

Looks good! The JIRA ticket doesn't explain very much so please correct me here, but the idea is to show this message even when a connection error occurs, right? Otherwise, do we still want to keep the ConnectionError value in the UploadSessionStatus enum along with its error message?

[cecilia-donnelly]

Yes, since the only specific error we look for is the out of storage error in upload.session.ts, right? I didn't see anywhere that we differentiate between a connection error and any other error (e.g. a zero byte file being uploaded, which is how I tested this). If I missed it, though, then I can definitely change this.

I originally opened the ticket in response to a concern from the experience side that users would see "connection error" and try to fix their connection (reasonable!) even though really something else was going on that made a file fail.