Make check_exists cache negative results as well as positive ones

When loading a workflow with URL references that can't be resolved,
don't keep trying them over and over again.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>

schema_salad/fetcher.py

@@ -108,7 +108,10 @@ def fetch_text(self, url: str, content_types: Optional[List[str]] = None) -> str
         raise ValidationException(f"Unsupported scheme in url: {url}")
 
     def check_exists(self, url: str) -> bool:
-        if url in self.cache:
+        entry = self.cache.get(url)
+        if entry is False:
+            return False
+        elif entry is not None:
             return True
 
         split = urllib.parse.urlsplit(url)
@@ -121,6 +124,7 @@ def check_exists(self, url: str) -> bool:
                 resp = self.session.head(url, allow_redirects=True)
                 resp.raise_for_status()
             except Exception:
+                self.cache[url] = False
                 return False
             self.cache[url] = True
             return True