Fix error handling in cmd

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.43478% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 87.40%. Comparing base (d59d485) to head (06e9e68).

Files	Patch %	Lines
cmd/ajson/main.go	0.00%	16 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #66 +/- ## ========================================== - Coverage 92.40% 87.40% -5.00% ========================================== Files 10 11 +1 Lines 2870 3034 +164 ========================================== Hits 2652 2652 - Misses 145 309 +164 Partials 73 73 ``` | [Flag](https://app.codecov.io/gh/spyzhov/ajson/pull/66/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stepan+Pyzhov) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/spyzhov/ajson/pull/66/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Stepan+Pyzhov) | `87.40% <30.43%> (-5.00%)` | :arrow_down: | 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=Stepan+Pyzhov#carryforward-flags-in-the-pull-request-comment) to find out more.

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