Change Canvas error message for canvas_api_permission_error

^{Originally posted by **mattdricker** February 23, 2022}
The current user-facing error shown when a student attempts to open a Canvas Files assignment and Hypothesis receives at `canvas_api_permission_error` is as follows:

```
Hypothesis couldn't get the assignment's file from Canvas.

You might not have permission to read the file in Canvas. This could be because:

* The file is marked as Unpublished in Canvas: an instructor needs to publish the file.
* This course was copied from another course: an instructor needs to edit this assignment and re-select the file.

If the problem persists, you can open a support ticket or visit our help documents.
```

### The Problem
This error can only be resolved by the instructor. However the error message shown to students only asks them to open a support ticket with Hypothesis. This results in the support team using additional time to write back to the student for contact info for the instructor before next reaching out directly to the instructor to troubleshoot, all for an issue that the student needn't be a primary participant in.

### The proposed solution
The text of the user-facing error message should be clearer that the student should first bring this to attention of their instructor, who can then reach out to Hypothesis support directly for assistance (if needed).

Proposed new text:
```
Hypothesis couldn't get the assignment's file from Canvas.

Please ask the instructor to check whether the file is:

* Unpublished
* Restricted
* Unavailable to students in this course

If the problem persists, you or your instructor can open a support ticket or visit our help documents.
```