Make `RSpec::Mocks::Constant.original(name)` handle invalid names gracefully.

Since this API returns an object that provides lots of details
about the constant (rather than just the original value), it's
not meant to raise errors when given constant names that can’t
resolve or are invalid -- instead the returned object indicates
that the provided name was invalid.
Merge pull request #833 from rspec/fix-constant-original-for-bad-const-names

Make `RSpec::Mocks::Constant.original(name)` handle invalid names gracefully.