Provide helpful error message for not providing C to heatmap

hvplot=0.8.1

related to https://github.com/holoviz/holoviews/issues/5444

I was trying to display a `heatmap` and I received an error message. I got it working and then during my refactor got the error message again. The error message was not indicative of the error in any way and I needed to debug into the hvplot code to understand the issue.

I.e. I wasted a lot of time.

The issue is that the script

```python
import plotly.express as px
import hvplot.pandas
import numpy as np
df=px.data.tips()
df["count"]=1
df.hvplot.heatmap(x="smoker", y="day", c="count")
```

gives the error

```bash
ValueError: Dimensions must be defined as a tuple, string, dictionary or Dimension instance, found a NoneType type.
```

The cause is the `c` instead `C`. Some kind of validation and helpful error message would be really appreciated here to make `hvplot` easy to use.