Don't fail when "from_name" cannot be found.

[docwhat]

if you set from_name but nothing matches it, then it raises and error instead of creating a new label (as if you had not set from_name).

As an example, if you setup this:

  name: ":pray: help wanted"
  color: "4caf50"
  description: "Extra attention is needed"
  from_name: "help wanted"

... but someone deleted "help wanted", then this action fails with:

Error: ❌ Label 'help wanted' not found. Cannot rename

This is merely annoying at the moment, but if #3 or #9 are completed (or if you have scripts that propagate GitHub configurations to multiple repositories) then this is a must have.

[crazy-max]

@docwhat Done, thanks for your input!