Manually error if path does not exist

For whatever reason, Julia < 1.2 on Windows does not error on realpath
if the path does not exist.  This is nonstandard behaviour, but the
behaviour nonetheless.  We need to try to construct the real path, and
then manually check if the path exists.

src/HiddenFiles.jl

@@ -190,6 +190,7 @@ end
 # Each OS branch defines its own _ishidden function.  In the main ishidden function, we check that the path exists, expand
 # the real path out, and apply the branch's _ishidden function to that path to get a final result
 function ishidden(f::AbstractString)
+    # If path does not exist, `realpath` will error™
     try
         rp = realpath(f)
     catch e
@@ -201,7 +202,13 @@ function ishidden(f::AbstractString)
         # If this fails for some other reason, rethrow
         rethrow()
     end
-    return _ishidden(f, realpath(f))
+
+    # Julia < 1.2 on Windows does not error on `realpath` if path does not exist, so we
+    # must do so manually here
+    ispath(rp) || throw(Base.uv_error("ishidden($(repr(f)))", Base.UV_ENOENT))
+
+    # If we got here, the path exists, and we can continue safely with our _ishidden checks
+    return _ishidden(f, rp)
 end