fix: it is better to return the error with c.Error

[aermolaev]

Describe the PR
If there is an error reading the doc.json file, then a 500 status code and context as plain/text is returned without any error information.

% curl -vs http://localhost:8080/api/swagger/doc.json
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET /api/swagger/doc.json HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.64.1
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Content-Type: text/plain; charset=utf-8
< X-Content-Type-Options: nosniff
< Date: Wed, 27 Oct 2021 12:10:47 GMT
< Transfer-Encoding: chunked
<
Internal Server Error

it is better to return the error using c.Error:

% curl -vs http://localhost:8080/api/swagger/doc.json
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> GET /api/swagger/doc.json HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.64.1
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Content-Type: application/json; charset=utf-8
< Date: Wed, 27 Oct 2021 12:11:07 GMT
< Transfer-Encoding: chunked
<
{"error":"not yet registered swag"}

[codecov]

Codecov Report

Merging #61 (83399cd) into master (f298ba3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #61   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           57        57           
=========================================
  Hits            57        57           

Impacted Files	Coverage Δ
swagger.go	100.00% <100.00%> (ø)

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[ubogdan]

LGTM

[ubogdan]

@aermolaev Thanks for your contribution.