Recently eporner slightly changed the URL layout, the ID that used to be
digits only are now digits and letters, so youtube-dl falls back to
the generic extractor that doesn't work.
Fix the matching regex to allow letters in ID.
[v2: added a test case]
* [bandcamp] raise ExtractorError when track not streamable
* [bandcamp] update md5 for second test
* don't rely on json-data, but just check for 'file'
* don't rely on presence of 'file'