lib.fileset: Make error messages more uniform

Just minor changes like:
- Always using "X is a Y, but it should be Z"
- "X is a path that does not exist" rather than "X does not exist"
- Always using multi-line strings for errors
- Always quoting string-like values and not quoting path-like values
  - But do quote filesystem roots. Even though they're paths, they might
    be very small, good to have quotes to know the start/end
- Capitalise the first word
- Distinguish root vs filesystem root more

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