Handle ImportError in version.opengl_vendor

Fixes #3698

doc/changelog.asciidoc

@@ -15,6 +15,15 @@ breaking changes (such as renamed commands) can happen in minor releases.
 // `Fixed` for any bug fixes.
 // `Security` to invite users to upgrade in case of vulnerabilities.
 
+v1.3.0 (unreleased)
+-------------------
+
+Fixed
+~~~~~
+
+- qutebrowser now starts properly when the PyQt5 QOpenGLFunctions package wasn't found.
+
+
 v1.2.0
 ------
 

qutebrowser/utils/version.py

@@ -453,7 +453,13 @@ def opengl_vendor():  # pragma: no cover
         vp = QOpenGLVersionProfile()
         vp.setVersion(2, 0)
 
-        vf = ctx.versionFunctions(vp)
+        try:
+            vf = ctx.versionFunctions(vp)
+        except ImportError as e:
+            log.init.debug("opengl_vendor: Importing version functions "
+                           "failed: {}".format(e))
+            return None
+
         if vf is None:
             log.init.debug("opengl_vendor: Getting version functions failed!")
             return None