check-snapshots: make exception for snapshot cache more targeted

The code in `check-snapshots` will print "No snapshots cache found
at ..." regardless of the error that happens when trying to open
the file. This can be misleading if e.g. the issue is permissions
to open the file or the file is corrupted. So make the exception
more targeted and only catch FileNotFound error and let python
how the full error for the other cases. Obviously this can be
done in many ways so I'm happy to tweak and e.g. keep catching
all exception but print the value etc.

tools/check-snapshots

@@ -23,7 +23,7 @@ def fetch_snapshots_api(url, timeout=SNAPSHOTS_TIMEOUT):
     start = time.time()
     try:
         r = requests.get(url, timeout=timeout)
-    except BaseException:  # pylint: disable=broad-exception-caught
+    except BaseException as e:  # pylint: disable=broad-exception-caught
         return None
     elapsed = time.time() - start
     if r.status_code != 200:
@@ -168,7 +168,7 @@ def main():
         try:
             with open(args.cache, encoding="utf8") as f:
                 snapshots = json.load(f)
-        except BaseException:  # pylint: disable=broad-exception-caught
+        except FileNotFoundError:
             print(f"No snapshots cache found at {args.cache}")
             sys.exit(1)
     else: