Throw error if path does not exist

Addresses #9.  This will hopefully avoid any problems that arise in
any internal functions used.  Note that further error handling should
be implemented, as, for example, checking for package or bundle still
seg faults when you give it the root directory.

src/HiddenFiles.jl

@@ -126,7 +126,10 @@ 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
-ishidden(f::AbstractString) = ispath(f) && _ishidden(realpath(f))
+function ishidden(f::AbstractString)
+    ispath(f) || throw(Base.uv_error("ishidden($(repr(f)))", Base.UV_ENOENT))
+    return _ishidden(realpath(f))
+end
 
 
 end