Use the same interface for vcs installation

flake8.main.git.install was already returning False if it couldn't
find the directory to install into. This makes mercurial.install do
the same thing and allows the vcs.install callback to understand that.

flake8/main/mercurial.py

@@ -44,8 +44,7 @@ def install():
     """Ensure that the mercurial hooks are installed."""
     hgrc = find_hgrc(create_if_missing=True)
     if hgrc is None:
-        print('Could not locate your root mercurial repository.')
-        raise SystemExit(True)
+        return False
 
     hgconfig = configparser_for(hgrc)
 
@@ -76,6 +75,8 @@ def install():
     with open(hgrc, 'w') as fd:
         hgconfig.write(fd)
 
+    return True
+
 
 def find_hgrc(create_if_missing=False):
     root = subprocess.Popen(

flake8/main/vcs.py

@@ -22,12 +22,16 @@ def install(option, option_string, value, parser):
     """
     installer = _INSTALLERS.get(value)
     errored = False
+    successful = False
     try:
-        installer()
+        successful = installer()
     except exc.HookInstallationError as hook_error:
         print(str(hook_error))
         errored = True
-    raise SystemExit(errored)
+
+    if not successful:
+        print('Could not find the {0} directory'.format(value))
+    raise SystemExit(not successful and errored)
 
 
 def choices():