fields.URL validation error should specify "not a valid FQDN" if trying to pass hostname without domain

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The current validation error message: `'Not a valid URL.'` is not descriptive enough to help the user, especially when they are passing a value that would appear to be a valid URL to most people, such as a docker hostname: `http://test-url:8001`

Sample code:
```py
from marshmallow import Schema, fields

class TestSchema(Schema):
url = fields.URL(required=True)

TestSchema().load({"url": "http://192.168"}) // this works
TestSchema().load({"url": "http://test-url"}) // this doesn't work
TestSchema().load({"url": "derp"}) // and this has the same error message as http://test-url
```

This is because fields.URL has a default: `require_tld=True`, for `require_tld: Whether to reject non-FQDN hostnames.`
FQDN = fully-qualified domain name. i.e. `http://server1.example.com` is a FQDN, `http://server1` is not.

Suggestion: error message should be more descriptive when require_tld is True, and validation fails FQDN requirement.

Start with fields.URL validation and the require_tld option described in the issue. Distinguish a non-FQDN hostname such as http://test-url from malformed input such as derp, then verify the sample cases and ensure valid FQDNs continue to pass.

Written by the indexing model from the issue text.