main: treat unhandled install operation warnings as errors by default

Signed-off-by: Filipe Laíns <lains@archlinux.org>

Author: FFY00

README.md

@@ -4,7 +4,7 @@ A simple, correct PEP427 wheel installer.
 
 ```sh
 $ python -m install -h
-usage: python -m install [-h] [--verbose] [--optimize [level [level ...]]] [--destdir /] [--cache] [--skip-build] [wheel]
+usage: python -m install [-h] [--verbose] [--optimize [level [level ...]]] [--destdir /] [--cache] [--skip-build] [--ignore-incomplete-installation-warnings] [wheel]
 
 positional arguments:
   wheel                 wheel file to install
@@ -17,6 +17,8 @@ optional arguments:
   --destdir /, -d /     destination directory
   --cache, -c           generate the installation cache
   --skip-build, -s      skip the cache building step, requires cache to be present already
+  --ignore-incomplete-installation-warnings, -w
+                        stop treating incomplete installation warnings as errors
 ```
 
 Missing components:

install/__main__.py

@@ -72,11 +72,18 @@ def _error(msg, code=1):  # type: (str, int) -> None
     parser.add_argument('--skip-build', '-s',
                         action='store_true',
                         help='skip the cache building step, requires cache to be present already')
+    # warnings
+    parser.add_argument('--ignore-incomplete-installation-warnings', '-w',
+                        action='store_true',
+                        help='stop treating incomplete installation warnings as errors')
     args = parser.parse_args()
 
     if args.verbose:
         logging.basicConfig(level=logging.DEBUG)
 
+    if not args.ignore_incomplete_installation_warning:
+        warnings.simplefilter('error', IncompleteInstallationWarning)
+
     cache_dir = '.install-cache'
     pkg_cache_dir = os.path.join(cache_dir, 'pkg')
     entrypoints_cache_dir = os.path.join(cache_dir, 'entrypoints')