file-system docs don't explain what errors are signaled

[cgay]

In general, the file-system docs don't seem to say what errors are signaled when functions like do-directory and directory-contents are called. There is an entry for <file-system-error> that say it's the "Error type signaled when any other functions in the File-System module signal an error". That's not good enough. Each function should say what error is signaled. Some are subclasses of <file-system-error>.

[cgay]

Relatedly, most errors in the file-system module result from just calling unix-file-error, which signals <file-system-error>. This results in what I consider to be bugs, like that calling file-property on a non-existent file doesn't result in a <file-does-not-exist-error>.