Mistyped urls can cause a stack trace

Background

If one doesn't enter a valid url in a .bst file and try to fetch it, urllib raises a ValueError which doesn't get handled, causing an ugly stack trace. E.g:

Failure Summary
  base/icu.bst:
    [00:00:00][a31d0083][fetch:base/icu.bst                  ] BUG     Fetch

        An unhandled exception occured:
    
        Traceback (most recent call last):
          File "/home/thomascoldrick/buildstream/buildstream/_scheduler/job.py", line 320, in _child_action
            result = self._action_cb(element)
          File "/home/thomascoldrick/buildstream/buildstream/_scheduler/fetchqueue.py", line 44, in process
            source._fetch()
          File "/home/thomascoldrick/buildstream/buildstream/source.py", line 378, in _fetch
            self.fetch()
          File "/home/thomascoldrick/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 90, in fetch
            sha256, _ = self._ensure_mirror()
          File "/home/thomascoldrick/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 100, in _ensure_mirror
            request = urllib.request.Request(self.url)
          File "/usr/lib/python3.5/urllib/request.py", line 269, in __init__
            self.full_url = url
          File "/usr/lib/python3.5/urllib/request.py", line 295, in full_url
            self._parse()
          File "/usr/lib/python3.5/urllib/request.py", line 324, in _parse
            raise ValueError("unknown url type: %r" % self.full_url)
        ValueError: unknown url type: 'https//github.com/winlibs/icu4c'

    [00:00:00][a31d0083][fetch:base/icu.bst                  ] BUG     Fetch

        An unhandled exception occured:
    
        Traceback (most recent call last):
          File "/home/thomascoldrick/buildstream/buildstream/_scheduler/job.py", line 320, in _child_action
            result = self._action_cb(element)
          File "/home/thomascoldrick/buildstream/buildstream/_scheduler/fetchqueue.py", line 44, in process
            source._fetch()
          File "/home/thomascoldrick/buildstream/buildstream/source.py", line 378, in _fetch
            self.fetch()
          File "/home/thomascoldrick/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 90, in fetch
            sha256, _ = self._ensure_mirror()
          File "/home/thomascoldrick/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 100, in _ensure_mirror
            request = urllib.request.Request(self.url)
          File "/usr/lib/python3.5/urllib/request.py", line 269, in __init__
            self.full_url = url
          File "/usr/lib/python3.5/urllib/request.py", line 295, in full_url
            self._parse()
          File "/usr/lib/python3.5/urllib/request.py", line 324, in _parse
            raise ValueError("unknown url type: %r" % self.full_url)
        ValueError: unknown url type: 'https//github.com/winlibs/icu4c'

    [00:00:00][a31d0083][fetch:base/icu.bst                  ] BUG     Fetch

        An unhandled exception occured:
    
        Traceback (most recent call last):
          File "/home/thomascoldrick/buildstream/buildstream/_scheduler/job.py", line 320, in _child_action
            result = self._action_cb(element)
          File "/home/thomascoldrick/buildstream/buildstream/_scheduler/fetchqueue.py", line 44, in process
            source._fetch()
          File "/home/thomascoldrick/buildstream/buildstream/source.py", line 378, in _fetch
            self.fetch()
          File "/home/thomascoldrick/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 90, in fetch
            sha256, _ = self._ensure_mirror()
          File "/home/thomascoldrick/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 100, in _ensure_mirror
            request = urllib.request.Request(self.url)
          File "/usr/lib/python3.5/urllib/request.py", line 269, in __init__
            self.full_url = url
          File "/usr/lib/python3.5/urllib/request.py", line 295, in full_url
            self._parse()
          File "/usr/lib/python3.5/urllib/request.py", line 324, in _parse
            raise ValueError("unknown url type: %r" % self.full_url)
        ValueError: unknown url type: 'https//github.com/winlibs/icu4c'

However I haven't managed to reproduce it, even without changing anything.