Warn when raise_error receives non-proc object

When a non-proc object is given, `given_proc.call` fails, then `raise_error` or
`raise_error(NoMethodError)` passes.

This behaviour is dangerous and there is actually no error, so it should warn
and fail.
Fix warning message
Merge branch '247'
Add a changelog entry for #247.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>