StandardError

        # StandardError < Exception

(from ruby core)
---
The most standard error types are subclasses of StandardError. A rescue
clause without an explicit Exception class will rescue all
StandardErrors (and only those).

    def foo
      raise "Oups"
    end
    foo rescue "Hello"   #=> "Hello"

On the other hand:

    require 'does/not/exist' rescue "Hi"

*raises the exception:*

    LoadError: no such file to load -- does/not/exist
---