Correct GridSpec override behavior

[kleavor]

Closes #514 .

Removed the 'warn' mode to keep only 'error' and 'override'.
'override' behavior now correctly removes the pre-existing Panel.

WIP since the object deletion feels like a kludge and an additional latent bug was noticed during 'error' behavior resulting from line 937-945.

https://github.com/kleavor/panel/blob/a5dff2e30cc08eee713d18ff6ac06d2f20e65c25/panel/layout.py#L937-L945

The if-statement on overlap only evaluates to true if the location of the new Panel object exactly matches the location of the conflicting Panel object. In the case of the example provided in the Issue, neither of the new panels matches the "Initial" Panel exactly, and the new object is still added despite erroring, since the error is not raised until the end of the method.

Behavior can be observed in a notebook with the following cells

pn.extension()
gs = pn.GridSpec(mode='error')
gs[0:2, 0] = pn.widgets.MultiSelect(options=[0, 1, 2, 3], name='Initial')
display(gs.objects)
gs

The following cell will raise an Exception.

gs[0:1, 0] = pn.widgets.MultiSelect(options=[5, 6, 7], name='Second')
gs[1:2, 0] = pn.widgets.MultiSelect(options=[9, 10, 11], name='Third')
display(gs.objects)

This cell will show that the state of the gs object now still contains the MultiSelect object despite erroring.

print(gs)

[philippjfr]

Thanks for helping out @kleavor! I hope you don't mind I fixed up the PR.