Improve the error messages for file I/O errors

[edmorley]

This improves the error messages shown for file related I/O errors, by:

• ensuring that the path is always included
• adding additional suggestions to the error based on the context (which is now known, since specialised types are used for each of the common scenarios like checking if a file exists or reading a file to a string)

We're able to make the errors richer than those provided by fs-err, since we know the context in which the errors have occurred.

For example, in the error message for ReadOptionalFileErrors we know not to say to check for a missing file or that the file is a directory, since read_optional_file() explicitly excludes those cases by design. If we used io::Error (or fs-err's equivalent) we would have to rely on the user not using the wrong logging function for the wrong I/O error case, rather than relying on the type system to do that for us.

These changes leave log_io_error() as being used only for the Command I/O cases. A later PR will switch adjust those usages too (but this PR is already large enough).

Closes #270.
GUS-W-12650236.