[entities] raise a ConfigurationError at registration time if no secret key is defined

entities.py

@@ -24,9 +24,16 @@ CONFENTRY = 'trustedauth-secret-key-file'
 # make sure the secret key file is loaded on both sides of cw (repo
 # and web)
 def registration_callback(vreg):
-    secret = open(vreg.config.get(CONFENTRY, "")).read().strip()
+    secretfile = vreg.config.get(CONFENTRY, "").strip()
+    if not secretfile:
+        raise ConfigurationError("Configuration '%s' is missing or empty. "
+                                 "Please check tour configuration file!" % CONFENTRY)
+    try:
+        secret = open(secretfile).read().strip()
+    except IOError:
+        raise ConfigurationError("Cannot open secret key file. Check your configuration file!")
     if not secret or len(secret) > 32:
         raise ConfigurationError('secret key must me a string 0 < len(key) <= 32')
-    
+
     vreg.config._secret = secret.ljust(32, '#')
     vreg.debug('loaded secret key')