fix: restore context after block.call

[iberianpig]

closes: #317

ref: iberianpig/fusuma-plugin-wmctrl#14 (comment)

If return is called in the with_context block, the context cannot be restored to its original state.
The "ensure" clause call post-processing even if it called returned in the middle of block.call.

The following problems were occurring:

1. First, config_params.fetch is called, :plugin_default_context is used as Context for the search
2. Then when Executor#executable? is called, the search is executed with :plugin_default_context
3. Because Executor#executable? returns false, wmctrl is not executed.