Set correct error message for ensure_loaded! in Plural

[maennchen]

Oops, that should not have been merged in #343

Copied from https://github.com/elixir-lang/elixir/blob/df056c2cda6c046839d42c830888c79c18287bfd/lib/elixir/lib/code.ex#L1648

[coveralls]

Pull Request Test Coverage Report for Build 23569650033868cfaad8fec145dd944d6e0fc172-PR-348

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.603%

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Totals
Change from base Build 7b350bc437786b63369d615dca25c173bc66ce2c:	0.0%
Covered Lines:	511
Relevant Lines:	564

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💛 - Coveralls

[whatyouhide]

Ah yep of course, good catch 👍