In certain scenarios, the front directory may not be writable.
When this is the case, writing the patched `index.html` should be
skipped just like when the file does not exist.
For brevity, I have replaced the `existsSync` check with a try-catch
that catches any errors occuring during read
or write of the index file.
The alternative would be `fs.accessSync` with `W_OK`, but that would
still throw an error if the file is not writable so I think it is
reasonable to skip it altogether and go straight for the read and write
attempts.
A specific scenario where the front directory is immutable is NixOS,
where the directory may be located in the read-only nix store.
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