_downloadablefilesource: handle ValueError-s

Description

Avoid unhandled exceptions like the following:

Traceback (most recent call last):
...
    File "/src/buildstream/buildstream/plugins/sources/_downloadablefilesource.py", line 108, in _ensure_mirror
    request = urllib.request.Request(self.url)
    File "/usr/lib/python3.6/urllib/request.py", line 329, in __init__
    self.full_url = url
    File "/usr/lib/python3.6/urllib/request.py", line 355, in full_url
    self._parse()
    File "/usr/lib/python3.6/urllib/request.py", line 384, in _parse
    raise ValueError("unknown url type: %r" % self.full_url)
ValueError: unknown url type: 'https//github.com/winlibs/icu4c.tar'

Explicitly handle ValueErrors, so that we report this condition in the usual way:

[00:00:00] FAILURE element.bst: tar source at
    element.bst [line 10 column 2]: Error mirroring
    https//github.com/winlibs/icu4c.tar: unknown url type:
    'https//github.com/winlibs/icu4c.tar'

Changes proposed in this merge request:

• Handle ValueError-s

This merge request, when approved, will close: #452 (closed)

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