RFR: 8220644: Align required/found pairs in diagnostics

[Liam Miller-Cushon]

This is a small whitespace adjustment to diagnostics that report a mismatch
between required/found arguments, which makes them a little easier to read.

webrev: http://cr.openjdk.java.net/~cushon/8220644/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8220644
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