watch targets can't be path-strings

[a11ce]

The quick-start programmatic example fails with a contract violation in path-on-disk because path? does not recognize path strings:

path-on-disk?: contract violation
  expected: path?
  given: "example.txt"
  in: the 1st argument of
      (-> path? boolean?)

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(define watcher (watch (list (string->path "example.txt")))) works correctly, and (define watcher (watch (list (string->path "does_not_exist.txt")))) fails with a correct error.