Fix Issue 21738 - std.format.spec: singleSpec should throw on "%%"

[wolframw]

singleSpec doesn't work with "%%" since it doesn't really "do" anything. Therefore, singleSpec is changed to throw a FormatException if "%%" is passed as format specifier.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21738	minor	std.format.spec: singleSpec should throw on "%%"

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7894"

[berni44]

You need to reword the commit message to start with "Fix issue 21738". So the bot will auto close the issue. All else is fine.

[RazvanN7]

Suggested change

	is malformed, or when the specifier is "%%".
	is malformed or when the specifier is "%%".

[berni44]

With #7891 being merged, this isn't relevant anymore. There are more cases, where a FormatException can be thrown, so I used a more general term there and there is no need any more to write anything about %%.

But: When handling the merge conflicts, the "Known Bug" section should be removed.

[wolframw]

Thanks for the heads-up.