Improved error message
minor #12598 Improved error message (WouterJ)

This PR was merged into the 2.7 branch.

Discussion
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Improved error message

If no alternatives could be found, the error message is: "Attempted to call method "getContents" on class "SebastianBergmann\Diff\Diff"." This doesn't point out any error, as it doesn't tell you that the method does not exists. This PR tries to fix this.

Commits
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5076a77 Improved error message