displayException: Remove trailing newline

[9999years]

In #36, I restructured the AnnotatedException displayException implementation so that instead of being of the form:

displayException =
    unlines [ ... ] <> callStackMessage
    where
        callStackMessage = prettyCallStack frames

It was of the form:

displayException =
    unlines $ [ ... ] <> callStackMessage
    where
        callStackMessage = [prettyCallStack frames]

I didn't realize that unlines adds a trailing newline, which is generally the wrong behavior for displayException instances.

This change replaces unlines with concat . intersperse "\n" in order to regain the correct behavior.

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