UTFException should derive from UnicodeException

[WalterBright]

UTFException should actually be replaced with UnicodeException, but there are some slight differences in behavior, sigh.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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dub fetch digger
dub run digger -- build "master + phobos#7145"

[thewilsonator]

'std/utf.d(63): Error: module std.exception import UnicodeException not found'