Add error handling for unsupported conversions

[dwojtaszek]

Is your feature request related to a problem? Please describe.
I tried to conver from JPG to PDF using Reubah pulled from git on 2/15/2025, when clicked Process Image nothing happened.
The container log has:

`>2025/02/05 10:24:15 Processing file: Resized_20250204_124059_1738748391601.jpg.jpeg, size: 24218 bytes

2025/02/05 10:24:15 Source format:

2025/02/05 10:24:15 Successfully decoded image as jpeg

2025/02/05 10:24:15 Error writing response: unsupported format: pdf

2025/02/05 10:24:15 POST /process 7.058447ms

2025/02/05 10:24:17 GET / 91.592µs

2025/02/05 10:24:21 Processing file: Resized_20250204_124059_1738748391601.jpg.jpeg, size: 24218 bytes

2025/02/05 10:24:21 Source format:

2025/02/05 10:24:21 Successfully decoded image as jpeg

2025/02/05 10:24:21 Error writing response: unsupported format: heic`

Describe the solution you'd like
An error popup to the user on the frontend or disabled unsupported formats on the dropdown list.

Describe alternatives you've considered
None

Additional context
This is user interface improvement.

[dendianugerah]

Hi, thanks for reporting this issue! It seems related to the last PR. I’ll look into it and push a fix soon

Adding an error popup or disabling unsupported formats in the dropdown makes sense—I’ll consider that as well. I’ll update you once it’s resolved

Thanks again!