Gracefully handle errors when failing to upload an attachment

[TassSinclair]

Context

As raised in #302, it's possible for attachment uploads to fail for reasons outside of Planka's control. However, Planka currently logs these errors to the console and continues displaying a spinner, leading the casual observer to assume that the upload is still continuing.

It would be better for Planka to intercept errors that occur during the upload process, and inform the user.

Scenarios

Focusing on 413 errors; I haven't reviewed the code so I'm not sure which errors are already handled, or what the error scenarios look like

• Attachment upload fails with 413 Request Entity Too Large
  GIVEN a Planka installation, served behind an NGINX reverse proxy
  AND the reverse proxy is using the default client max body size (1MB)
  WHEN the user uploads a 2MB attachment to a card
  AND a spinner shows on the card to represent the attachment upload
  AND the server responds with a 413 Request Entity Too Large
  THEN an error is shown to the user, alerting them of the failure (how? toast? A dialog?)
  AND the spinner is dismissed
• Attachment upload failure message has link to documentation
  GIVEN a Planka installation, served behind an NGINX reverse proxy
  AND the reverse proxy is using the default client max body size (1MB)
  AND the user uploads a 2MB attachment to a card
  AND the server responds with a 413 Request Entity Too Large
  THEN the error shown includes a link to documentation (wiki? faq?) with suggestions of how to debug the issue.

[meltyshev]

Hi! Thanks for reporting this. This really needs to be fixed, I think it will be in the v2 release (if no one wants to fix this sooner via PR).