`IRB::Inspector#inspect_value` errors
(https://github.com/ruby/irb/pull/511)
**Before**
```
irb(main):001:0> c = Cat.new "foo"
(Object doesn't support #inspect)
=>
```
**After**
```
irb(main):001:0> c = Cat.new "foo"
An error occurred when inspecting the object: #<NoMethodError: undefined method `is_a?' for foo:Cat
if obj.is_a?(String)
^^^^^^>
Result of Kernel#inspect: #<Cat:0x0000000109090d80 @name="foo">
=>
```
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