Fix Issue 21707 - std.base64: Faulty input creates range error instead of Base64Exception

[berni44]

Removed a misleading sentence from the documentation too (my last attempt to fix this, was guided by this sentence...)

Probably this fix is also needed for the unpadded version, but I don't know this exactly yet and don't want to break there anything...

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21707	normal	std.base64: Faulty input creates range error instead of Base64Exception

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#7871"