Extract error message from API response for preview.

[sharief007]

Change Log

• For preview errors, only isErrorShown is set to true and errorMessage is not provided. Hence the comment form shows Something went wrong. Please try again a bit later. (generic message) by default for all preview errors.
• Refactored the exception handling to derive error message from the response code (similar to send method). Now both Send and Preview shows the same error message.

Resolves: https://github.com/umputun/remark42/issues/1673

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c9b6f92) 61.55% compared to head (879db73) 61.55%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1674 +/- ## ======================================== Coverage 61.55% 61.55% ======================================== Files 133 133 Lines 3004 3004 Branches 752 759 +7 ======================================== Hits 1849 1849 - Misses 1038 1151 +113 + Partials 117 4 -113 ``` | [Files Changed](https://app.codecov.io/gh/umputun/remark42/pull/1674?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun) | Coverage Δ | | |---|---|---| | [...ark42/app/components/comment-form/comment-form.tsx](https://app.codecov.io/gh/umputun/remark42/pull/1674?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun#diff-ZnJvbnRlbmQvYXBwcy9yZW1hcms0Mi9hcHAvY29tcG9uZW50cy9jb21tZW50LWZvcm0vY29tbWVudC1mb3JtLnRzeA==) | `53.30% <0.00%> (ø)` | | ... and [23 files with indirect coverage changes](https://app.codecov.io/gh/umputun/remark42/pull/1674/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun)

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[paskal]

Thanks a lot! I've checked now and it works as expected.