Validate plot backend when setting.
```python
In [1]: import pandas as pd

In [2]: import sys

In [3]: import types

In [4]: module = types.ModuleType("foo")

In [5]: sys.modules['foo'] = module

In [6]: pd.set_option('plotting.backend', 'foo')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-123b6513deb2> in <module>
----> 1 pd.set_option('plotting.backend', 'foo')

...

~/sandbox/pandas/pandas/plotting/_core.py in _find_backend(backend)
   1588         "top-level `.plot` method."
   1589     )
-> 1590     raise ValueError(msg.format(name=backend))
   1591
   1592

ValueError: Could not find plotting backend 'foo'. Ensure that you've installed the package providing the 'foo' entrypoint, or that the package has atop-level `.plot` method.

```

Closes https://github.com/pandas-dev/pandas/issues/28163
cc @datapythonista Merging later today if there aren't any objections. LGTM