Auto merge of #44754 - marcusbuffett:bootstrap-config-toml-fix, r=alexcrichton

Catch IOError in bootstrap.py when loading config.toml

When I pulled this repo and tried to build using the command in the readme, I got an error about a missing `config.toml`.

If config.toml doesn't exist, then an `IOError` will be raised the `with open(...)` line. Prior to e788fa7b6c, this was caught because the `except` clause didn't specify what exceptions it caught, so both `IOError` and `OSError` were caught.

First time contributing, so please let me know if I'm doing anything wrong.

src/bootstrap/bootstrap.py

@@ -682,7 +682,7 @@ def bootstrap():
     try:
         with open(args.config or 'config.toml') as config:
             build.config_toml = config.read()
-    except OSError:
+    except (OSError, IOError):
         pass
 
     if '\nverbose = 2' in build.config_toml: