Make hv.Cycle KeyError text more helpful or read keys for hv.Cycle automatically

[fohrloop]

Using: Holoviews 1.14.8

Is your feature request related to a problem? Please describe.

When following some examples from the internet, there might be a point where hv.Cycle is used. Usage of it needs import with side effects. Without any imports, user will see:

In [1]: import holoviews as hv
   ...: hv.Cycle("Set1")
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Input In [1], in <module>
      1 import holoviews as hv
----> 2 hv.Cycle("Set1")

File c:\python\venvs\myproj\lib\site-packages\holoviews\core\options.py:325, in Cycle.__init__(self, cycle, **params)
    323         params['key'] = None
    324 super(Cycle, self).__init__(**params)
--> 325 self.values = self._get_values()

File c:\python\venvs\myproj\lib\site-packages\holoviews\core\options.py:335, in Cycle._get_values(self)
    333 if self.values: return self.values
    334 elif self.key:
--> 335     return list(self.default_cycles[self.key])
    336 else:
    337     raise ValueError("Supply either a key or explicit values.")

KeyError: 'Set1'

While the following works (as calling hv.extension has a side effect that is needed for hv.Cycle):

In [1]: import holoviews as hv
   ...: hv.extension()
   ...:
   ...: hv.Cycle("Set1")
Out[1]: Cycle('Set1')

Describe the solution you'd like

Instead of printing

KeyError: 'Set1'

print

KeyError: 'Set1'. Did you forget to call panel.extension?

Describe alternatives you've considered

Another possibility could be to call hv.extension, or somehow else fetch the default list of keys for hv.Cycle, in case of error.

---

Edit: replaced pn.extension with hv.extension.

[github-actions]

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