fix(scram): allow errors to be passed through callbacks

Fixes NODE-1876

Author: kvwalker

lib/auth/scram.js

@@ -143,7 +143,11 @@ class ScramSHA extends AuthProvider {
 
       processedPassword = saslprep ? saslprep(password) : password;
     } else {
-      processedPassword = passwordDigest(username, password);
+      try {
+        processedPassword = passwordDigest(username, password);
+      } catch (e) {
+        return callback(e);
+      }
     }
 
     // Clean up the user