{Pairing, Housekeeping, RM} API: Handle `Bad Request`s

[Annopaolo]

Do not return an error 400 with message "Internal server error" if a request has an unparsable payload (e.g. a badly written CSR). Return the more semantically correct "Bad request" message instead.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (210eca0) 68.72% compared to head (cd3f480) 68.68%.

Files	Patch %	Lines
...b/astarte_housekeeping_api_web/views/error_view.ex	0.00%	1 Missing :warning:
...pi/lib/astarte_pairing_api_web/views/error_view.ex	0.00%	1 Missing :warning:
...tarte_realm_management_api_web/views/error_view.ex	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.0 #894 +/- ## =============================================== - Coverage 68.72% 68.68% -0.04% =============================================== Files 247 247 Lines 5550 5553 +3 =============================================== Hits 3814 3814 - Misses 1736 1739 +3 ```

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