fix(pngquant): handle malformed png file errors

## Steps To Reproduce The Error

```sh
❯ echo "hello" > file.png
❯ imageoptim --imagealpha file.png
i Running ImageAlpha...
! Command failed with exit code 2 (ENOENT): /Applications/ImageAlpha.app/Contents/MacOS/pngquant --ext=.png --force --skip-if-larger --speed=1 --quality=65-80 256 -- /var/folders/57/f83bftsx79b9ylc2gzbdmdfw0000gp/T/imageoptim-cli/file.png

! Please raise an issue at https://github.com/JamieMason/ImageOptim-CLI/issues

Error: Command failed with exit code 2 (ENOENT): /Applications/ImageAlpha.app/Contents/MacOS/pngquant --ext=.png --force --skip-if-larger --speed=1 --quality=65-80 256 -- /var/folders/57/f83bftsx79b9ylc2gzbdmdfw0000gp/T/imageoptim-cli/file.png
at makeError (/usr/local/Cellar/imageoptim-cli/3.0.2/libexec/lib/node_modules/imageoptim-cli/dist/imageoptim:6571:11)
at handlePromise (/usr/local/Cellar/imageoptim-cli/3.0.2/libexec/lib/node_modules/imageoptim-cli/dist/imageoptim:6368:26)
at process._tickCallback (internal/process/next_tick.js:68:7)
```

## Expected Behaviour

ignore or report malformed png files

## Actual Behaviour

crash

## Version Numbers or N/A

- macOS: 10.15.7
- /Applications/ImageOptim.app: 1.8.8 (brew)
- /Applications/ImageAlpha.app: 1.5.1 (brew)
- /Applications/JPEGmini.app: N/A

## Help Needed

would the test at https://github.com/JamieMason/ImageOptim-CLI/blob/e65bb4a24c23975297aac716cf77fc4b87827ef9/src/is-supported.ts#L4 be complemented by a magic number test on the actual file's contents?

Start with the test at src/is-supported.ts#L4 and reproduce the malformed file case using the commands in the issue. Check how the CLI handles the pngquant failure, then verify that malformed PNG files are ignored or reported instead of crashing.

Written by the indexing model from the issue text.