An error whose existence depends on files other than the one being analysed - a path named in the
code that has to exist, a data file read at analysis time. The result cache watches the files
listed here and re-analyses the file the error is reported in when one of them appears, changes or
is deleted, which nothing else would do: the dependency graph is built from reflected symbols, and
a path is not a symbol. Paths must be absolute and are watched whether or not they exist, so an error about a missing file
is cleared by that file being created.