[GRASS-git] [OSGeo/grass] befb07: g.gui.timeline: improve missing package msg

[Markus Neteler]

  Branch: refs/heads/releasebranch_7_8
  Home:   https://github.com/OSGeo/grass
  Commit: befb07394202924f4a5c8b914562ed108598f467
      https://github.com/OSGeo/grass/commit/befb07394202924f4a5c8b914562ed108598f467
  Author: Markus Neteler <neteler at gmail.com>
  Date:   2020-07-25 (Sat, 25 Jul 2020)

  Changed paths:
    M gui/wxpython/timeline/frame.py

  Log Message:
  -----------
  g.gui.timeline: improve missing package msg

Be less obscure with the error:

```
Unable to start Timeline Tool.
Reason: The Timeline Tool needs the "matplotlib" (python-rmatplotlib)
package to be installed. No module named
‘mmatplotlib.backends.backend_wxagg'
...
```