Fix the exception message when validating unicode URLs

[wojcikstefan]

Prior to this PR, formatting the exception message would fail:

In [1]: from mongoengine.fields import URLField

In [2]: URLField().validate(u'http://привет.com')
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-2-023c2c193da6> in <module>()
----> 1 URLField().validate(u'http://привет.com')

/Users/wojcikstefan/Repos/temp/mongoengine/mongoengine/fields.py in validate(self, value)
    145         # Then check full URL
    146         if not self.url_regex.match(value):
--> 147             self.error('Invalid URL: {}'.format(value))
    148             return
    149

UnicodeEncodeError: 'ascii' codec can't encode characters in position 7-12: ordinal not in range(128)