Refactor MqttError type

[codecov[bot]]

Codecov Report

Patch coverage: 18.18% and project coverage change: -0.83% :warning:

Comparison is base (f874e8b) 65.36% compared to head (f976188) 64.54%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #149 +/- ## ========================================== - Coverage 65.36% 64.54% -0.83% ========================================== Files 53 53 Lines 9289 9366 +77 ========================================== - Hits 6072 6045 -27 - Misses 3217 3321 +104 ``` | [Files Changed](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs) | Coverage Δ | | |---|---|---| | [src/error.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL2Vycm9yLnJz) | `25.42% <0.00%> (-0.90%)` | :arrow_down: | | [src/server.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3NlcnZlci5ycw==) | `61.84% <0.00%> (-1.27%)` | :arrow_down: | | [src/types.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3R5cGVzLnJz) | `5.26% <0.00%> (-44.74%)` | :arrow_down: | | [src/v3/publish.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3YzL3B1Ymxpc2gucnM=) | `6.12% <0.00%> (ø)` | | | [src/v3/selector.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3YzL3NlbGVjdG9yLnJz) | `0.00% <0.00%> (ø)` | | | [src/v3/shared.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3YzL3NoYXJlZC5ycw==) | `69.07% <0.00%> (-2.30%)` | :arrow_down: | | [src/v5/codec/codec.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3Y1L2NvZGVjL2NvZGVjLnJz) | `68.70% <0.00%> (-3.96%)` | :arrow_down: | | [src/v5/selector.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3Y1L3NlbGVjdG9yLnJz) | `0.00% <0.00%> (ø)` | | | [src/v5/shared.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3Y1L3NoYXJlZC5ycw==) | `69.50% <0.00%> (-1.77%)` | :arrow_down: | | [src/v3/server.rs](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs#diff-c3JjL3YzL3NlcnZlci5ycw==) | `57.50% <7.14%> (-0.42%)` | :arrow_down: | | ... and [9 more](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs) | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/ntex-rs/ntex-mqtt/pull/149/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ntex-rs)

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