only warn when there is an invalid path when getting a prop

collects/meta/props

@@ -140,7 +140,7 @@ path/s is either such a string or a list of them.
   ;; need updating if more characters are allowed in the future.
   #rx"[^/.a-zA-Z0-9%_+-]")
 
-(define (validate-path-string path-string who)
+(define (validate-path-string path-string who [only-warn? #f])
   (define (bad why)
     (error* who "invalid path argument, expecting a ~a, got: ~e"
             why path-string))
@@ -149,10 +149,12 @@ path/s is either such a string or a list of them.
              (regexp-match? rx:bad-path path-string))
     (bad "relative `/'-delimited string, no `/' suffix, `//', `.', or `..'"))
   (when (regexp-match? rx:bad-pathchar path-string)
-    (error* who "invalid path argument, ~s is not allowed, got: ~e\n~a~a"
-            (regexp-match rx:bad-pathchar path-string) path-string
-            "(note: if paths with this character are needed, then this"
-            " script needs to be exteded to allow them)")))
+    (if only-warn?
+      (warn "~s is a bad path argument" path-string)
+      (error* who "invalid path argument, ~s is not allowed, got: ~e\n~a~a"
+              (regexp-match rx:bad-pathchar path-string) path-string
+              "(note: if paths with this character are needed, then this"
+              " script needs to be exteded to allow them)"))))
 
 (define (parse-prop-string prop str who)
   (with-handlers ([exn? (lambda (e)
@@ -162,7 +164,7 @@ path/s is either such a string or a list of them.
 
 (define (get-prop path-string prop-name [default get-prop]
                   #:strict? [strict? #f] #:as-string? [as-string? #f])
-  (validate-path-string path-string 'get-prop)
+  (validate-path-string path-string 'get-prop #t) ; no errors
   (let ([upchain
          ;; take the chain going up from the most specific node, so that
          ;; properties of a directory apply to subpaths