pdfjs: Fix :version crash with no pdf.js

pdfjs.get_pdf_basename() returned None, causing in a TypeError. Instead of
throwing mocker.patch at it, fix the underlying issue.

Given we made the same mistake in three places:

- :version
- test_real_file for PDF.js
- is_available() in pdfjs.py (calls the function but doesn't use the result, so
  is a nop now, even if PDF.js wasn't found)

...evidently we need to change the API so it still raises an exception if no
PDF.js is available.

Amends 0144ae3576.

qutebrowser/browser/pdfjs.py

@@ -212,7 +212,8 @@ def get_pdfjs_basename():
             pass
         else:
             return ext
-    return None
+
+    raise PDFJSNotFound(" or ".join(f"'build/{ext}'" for ext in exts))
 
 
 def is_available():

tests/unit/utils/test_version.py

@@ -849,13 +849,11 @@ class TestPDFJSVersion:
     """Tests for _pdfjs_version."""
 
     def test_not_found(self, mocker):
-        mocker.patch('qutebrowser.utils.version.pdfjs.get_pdfjs_basename')
         mocker.patch('qutebrowser.utils.version.pdfjs.get_pdfjs_res_and_path',
                      side_effect=pdfjs.PDFJSNotFound('/build/pdf.js'))
         assert version._pdfjs_version() == 'no'
 
-    def test_unknown(self, monkeypatch, mocker):
-        mocker.patch('qutebrowser.utils.version.pdfjs.get_pdfjs_basename')
+    def test_unknown(self, monkeypatch):
         monkeypatch.setattr(
             'qutebrowser.utils.version.pdfjs.get_pdfjs_res_and_path',
             lambda path: (b'foobar', None))
@@ -880,7 +878,6 @@ class TestPDFJSVersion:
               // Use strict in our context only - users might not want it
               'use strict';
         """.replace('VARNAME', varname)).strip().encode('utf-8')
-        mocker.patch('qutebrowser.utils.version.pdfjs.get_pdfjs_basename')
         monkeypatch.setattr(
             'qutebrowser.utils.version.pdfjs.get_pdfjs_res_and_path',
             lambda path: (pdfjs_code, '/foo/bar/pdf.js'))