hooks: Fail ReadDir if a configured hook executable is missing

The continue here is from 5676597f (hooks/read: Ignore IsNotExist for
JSON files in ReadDir, 2018-04-27, #686), where it was intended to
silently ignore missing JSON files.  However, the old logic was also
silently ignoring not-exist errors from the os.Stat(hook.Hook.Path)
from 68eb128f (pkg/hooks: Version the hook structure and add 1.0.0
hooks, 2018-04-27, #686).  This commit adjusts the check so JSON
not-exist errors continue to be silently ignored while hook executable
not-exist errors become fatal.

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #887
Approved by: rhatdan

pkg/hooks/read.go

@@ -67,13 +67,16 @@ func ReadDir(path string, extensionStages []string, hooks map[string]*current.Ho
 	}
 
 	for _, file := range files {
-		hook, err := Read(filepath.Join(path, file.Name()), extensionStages)
+		filePath := filepath.Join(path, file.Name())
+		hook, err := Read(filePath, extensionStages)
 		if err != nil {
 			if err == ErrNoJSONSuffix {
 				continue
 			}
 			if os.IsNotExist(err) {
-				continue
+				if err2, ok := err.(*os.PathError); ok && err2.Path == filePath {
+					continue
+				}
 			}
 			return err
 		}