Ensure ImportError's have a message

Otherwise, you can get, e.g., a test skip with an empty message like:
```
lib/matplotlib/tests/test_getattr.py::test_getattr[matplotlib.backends.backend_gtk3] SKIPPED (Cannot import matplotlib.backends.backend_gtk3 due to)
```

Author: QuLogic

lib/matplotlib/backends/backend_cairo.py

@@ -15,7 +15,8 @@
 try:
     import cairo
     if cairo.version_info < (1, 14, 0):  # Introduced set_device_scale.
-        raise ImportError
+        raise ImportError(f"Cairo backend requires cairo>=1.14.0, "
+                          f"but only {cairo.version_info} is available")
 except ImportError:
     try:
         import cairocffi as cairo

lib/matplotlib/backends/backend_gtk3.py

@@ -21,7 +21,7 @@
 except ValueError as e:
     # in this case we want to re-raise as ImportError so the
     # auto-backend selection logic correctly skips.
-    raise ImportError from e
+    raise ImportError(e) from e
 
 from gi.repository import Gio, GLib, GObject, Gtk, Gdk
 from . import _backend_gtk

lib/matplotlib/backends/backend_gtk4.py

@@ -20,7 +20,7 @@
 except ValueError as e:
     # in this case we want to re-raise as ImportError so the
     # auto-backend selection logic correctly skips.
-    raise ImportError from e
+    raise ImportError(e) from e
 
 from gi.repository import Gio, GLib, Gtk, Gdk, GdkPixbuf
 from . import _backend_gtk