Raise on attempting to remove a not existing child

[FelonEkonom]

Related Jira ticket: https://membraneframework.atlassian.net/browse/MC-176

[mat-hek]

Suggested change

	|> List.delete(nil)
	|> Enum.uniq()
	|> Enum.uniq()
	|> List.delete(nil)

[mat-hek]

instead of searching through all children, we could just search in the ones that are to be removed

[FelonEkonom]

value refs in line 522 contains only references to the children that we have to remove, while variable children_or_children_groups might contain not only children refs, but child groups names as well, so I am not convinced if this change will make the code simpler

[mat-hek]

but we can easily get the groups when retrieving refs

[FelonEkonom]

I have moved part of code checking whether removed child exist to the another function, IMO this is cleaner then previous version

[mat-hek]

Suggested change

	Enum.find(removed_children_or_groups, fn name ->
	not Map.has_key?(state.children, name) and not MapSet.member?(children_groups, name)
	end)
	|> case do
	nil ->
	:ok
	child_ref ->
	raise Membrane.ParentError, """
	Trying to remove child #{inspect(child_ref)}, while such a child or children group does not exist.
	removed_children_or_groups
	|> Enum.reject(fn name ->
	Map.has_key?(state.children, name) or MapSet.member?(children_groups, name)
	end)
	|> case do
	[] ->
	:ok
	children_refs ->
	raise Membrane.ParentError, """
	Trying to remove children #{Enum.map_join(", ", &inspect/1)}, while such children or children groups do not exist.