Improve error messaging around invalid characters in import

[marcusmoore]

Description

This PR improves the error messaging around characters that cannot be encoded during the beginning of the import process.

Currently, if an import contains a character that cannot be encoded to JSON in the header or first content rows the status bar turns red but the user is not given a message telling them what went wrong:

With this PR, a message is shown to the user indicating what happened and pointing them to either the header or content row:

This also includes returning 422 status codes when the import is in an invalid state instead of 500 and correcting the store method's return type.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

This was tested by pasting an invalid 3 octet sequence into a field in the header and first content rows of an import template.
The invalid character I used was: "\xe2\x28\xa1" (â(¡)

[what-the-diff]

• The file type check was changed from a 500 to 422 error.
• A new exception handler for malformed UTF-8 characters in the header row and first data row were added, returning a 422 response code instead of an internal server error (500).

[snipe]

Awesome, thank you!