update error msg

lib/eckles.js

@@ -336,11 +336,11 @@ EC.exportSync = function (opts) {
     if (!format || 'sec1' === format) {
       format = 'sec1';
     } else if ('pkcs8' !== format) {
-      throw new Error("options.format must be 'sec1' or 'pkcs8' for private EC keys");
+      throw new Error("options.format must be 'sec1' or 'pkcs8' for private EC keys, not '" + format + "'");
     }
   }
   if (-1 === [ 'P-256', 'P-384' ].indexOf(jwk.crv)) {
-    throw new Error("options.jwk.crv must be either P-256 or P-384 for EC keys");
+    throw new Error("options.jwk.crv must be either P-256 or P-384 for EC keys, not '" + jwk.crv + "'");
   }
   if (!jwk.y) {
     throw new Error("options.jwk.y must be a urlsafe base64-encoded either P-256 or P-384");