[common] simplify HTTP error messages

[warning] HTTPSConnectionPool(host='domain.tld', port=443): Max retries
exceeded with url: /a.jpg (Caused by NameResolutionError("<urllib3.
connection.HTTPSConnection object at 0x7247fe436ea0>: Failed to resolve
'domain.tld' ([Errno -2] Name or service not known)")) (1/5)

->

[warning] NameResolutionError: Failed to resolve 'domain.tld'
([Errno -2] Name or service not known) (1/5)

gallery_dl/downloader/http.py

@@ -144,7 +144,16 @@ class HttpDownloader(DownloaderBase):
                     proxies=self.proxies,
                     verify=self.verify,
                 )
-            except (ConnectionError, Timeout) as exc:
+            except ConnectionError as exc:
+                try:
+                    reason = exc.args[0].reason
+                    cls = reason.__class__.__name__
+                    pre, _, err = str(reason.args[-1]).partition(":")
+                    msg = "{}: {}".format(cls, (err or pre).lstrip())
+                except Exception:
+                    msg = str(exc)
+                continue
+            except Timeout as exc:
                 msg = str(exc)
                 continue
             except Exception as exc: