Fix lack of error on file site export, improve lack of ADMIN_PASSWD error message

[paskal]

Previously, status 200 was set for file export, which is used for backup, which resulted in an inability to set an error status code in case of a problem with file generation.

After this change, status code 200 would be written automatically by Go before we start writing the response's body.

Also, add meaningful error for lack of auth on import, remap and backup.

Previously, the error printed was just the following:

error response "401 Unauthorized", Unauthorized"

New error:

error response "401 Unauthorized", ensure you have set ADMIN_PASSWD and provided it to the command you're running: Unauthorized

[github-actions[bot]]

Pull Request Test Coverage Report for Build 7686355908

• 0 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 84.878%

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Files with Coverage Reduction	New Missed Lines	%
backend/app/rest/api/rss.go	1	83.33%
<!--	Total:	1		-->

Totals
Change from base Build 7596391691:	0.1%
Covered Lines:	5972
Relevant Lines:	7036

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💛 - Coveralls