Add debug string to assertion error message.

[olafurpg]

This commit changes the assertion error messaage in #2797 from an empty
message to:

found:    Array[java.nio.file.FileVisitOption]
[error] required: java.nio.file.FileVisitOption

[dottybot]

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Commit Messages

We want to keep history, but for that to actually be useful we have
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Please stick to these guidelines for commit messages:

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adapted from https://chris.beams.io/posts/git-commit

Have an awesome day! ☀️

[odersky]

LGTM