Ideas for backwards-compatible solutions (none of them particularly clean):
ignore could pass more arguments to the block, which is backwards-compatible assuming clients use a proc.
Add a new method ignore2 which receives observation objects instead of values.
Pass a custom nil-value as observation value to ignore if an exception is raised. This special nil-value should expose the exception.
Ideas for backwards-compatible solutions (none of them particularly clean):
ignore could pass more arguments to the block, which is backwards-compatible assuming clients use a proc. Add a new method ignore2 which receives observation objects instead of values. Pass a custom nil-value as observation value to ignore if an exception is raised. This special nil-value should expose the exception.