billing/sponsorship: Allow blank organization URLs.

We ran into a bug in production caused by two issues:
- Some users came from orgs that didn't have a website and since
  the URL field was required, they submitted invalid URLs.
- We didn't properly respond to invalid form submissions, which
  led to UnboundLocalError exceptions in another part of the
  code.

This commit solves this by doing the following:
- We now allow blank URLs and have a convenient placeholder text
  label that tells users that they may leave the URL field blank.
- This commit refactors the code such that invalid form submissions
  result in an informative error message about what exactly went
  wrong.

templates/corporate/upgrade.html

@@ -238,7 +238,7 @@
                                 <label>
                                     <h4>Organization website</h4>
                                 </label>
-                                <input name="website" style="width: 100%;" type="text" class="input-large" required/>
+                                <input name="website" style="width: 100%;" type="text" class="input-large" placeholder="{{ _('Leave blank if your organization does not have a website.') }}"/>
                                 <label>
                                     <h4>Describe your organization briefly</h4>
                                 </label>