refactor(server): use more specific HTTP status codes

Description

This change returns a more appropriate status code in 2 cases instead of a generic 500 code:

UnsupportedMediaType (if the mime type is on the blacklist)
Conflict (if the file already exists)

Motivation and Context

closes #261

How Has This Been Tested?

cargo test
fixtures

Changelog Entry

### Changed

- Return appropriate HTTP status code instead of 500

Types of Changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Documentation (no code change)
[x] Refactor (refactoring production code)
[ ] Other

Checklist:

[x] My code follows the code style of this project.
[ ] I have updated the documentation accordingly.
[x] I have formatted the code with rustfmt.
[x] I checked the lints with clippy.
[x] I have added (in this case changed) tests to cover my changes.
[x] All new and existing tests passed.

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.35%. Comparing base (4987cfe) to head (075fc84).

Files	Patch %	Lines
src/paste.rs	60.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #262 +/- ## ========================================== + Coverage 82.32% 82.35% +0.02% ========================================== Files 11 11 Lines 1228 1224 -4 ========================================== - Hits 1011 1008 -3 + Misses 217 216 -1 ``` | [Flag](https://app.codecov.io/gh/orhun/rustypaste/pull/262/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | Coverage Δ | | |---|---|---| | [unit-tests](https://app.codecov.io/gh/orhun/rustypaste/pull/262/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | `82.35% <60.00%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z#carryforward-flags-in-the-pull-request-comment) to find out more.

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orhun / rustypaste