Infinite recursion when displaying an exception cause

[cbandy]

It is possible for two exceptions to refer to each other as their cause, resulting in infinite recursion in display_exception_details. Here is a snippet that demonstrates the case:

irb(main):001:0> begin
irb(main):002:1*   begin
irb(main):003:2*     require 'nope_a'
irb(main):004:2>   rescue LoadError => e
irb(main):005:2>     begin
irb(main):006:3*       require 'nope_b'
irb(main):007:3>     rescue LoadError
irb(main):008:3>       raise e
irb(main):009:3>     end
irb(main):010:2>   end
irb(main):011:1> rescue Exception => f
irb(main):012:1>   a = f
irb(main):013:1> end
=> #<LoadError: cannot load such file -- nope_a>
irb(main):014:0> a.cause
=> #<LoadError: cannot load such file -- nope_b>
irb(main):015:0> a.cause.cause
=> #<LoadError: cannot load such file -- nope_a>
irb(main):016:0> a.cause.cause.cause
=> #<LoadError: cannot load such file -- nope_b>
irb(main):017:0> a.cause.cause.cause.cause
=> #<LoadError: cannot load such file -- nope_a>

I discovered this while using Sequel in a Rake task. That gem uses this construct to load drivers in a particular order and raise the preferred driver when all fail. I'm going to recommend a change over in that project, but this was so easy to replicate, I thought Rake may need to guard against it as well.