[Intl] Improve the error message when country code is wrong

Author: javiereguiluz

Tests/TimezonesTest.php

@@ -603,6 +603,15 @@ public function testForCountryCodeWithUnknownCountry()
         Timezones::forCountryCode('foobar');
     }
 
+    /**
+     * @expectedException \Symfony\Component\Intl\Exception\MissingResourceException
+     * @expectedExceptionMessage Country codes must be in uppercase, but "nl" was passed. Try with "NL" country code instead.
+     */
+    public function testForCountryCodeWithWrongCountryCode()
+    {
+        Timezones::forCountryCode('nl');
+    }
+
     /**
      * @expectedException \Symfony\Component\Intl\Exception\MissingResourceException
      */

Timezones.php

@@ -109,6 +109,10 @@ public static function forCountryCode(string $country): array
                 return [];
             }
 
+            if (Countries::exists(strtoupper($country))) {
+                throw new MissingResourceException(sprintf('Country codes must be in uppercase, but "%s" was passed. Try with "%s" country code instead.', $country, strtoupper($country)));
+            }
+
             throw $e;
         }
     }