Better error message for wrong encoding

[ankane]

Hi, thanks for this neat library! This improves the error message when the ciphertext has the wrong encoding. The current message is IV must be Encoding::BINARY, which makes it seem like the nonce instead of the ciphertext.

	3: from /Users/andrew/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/miscreant-0.3.0/lib/miscreant/aead.rb:90:in `open'
	2: from /Users/andrew/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/miscreant-0.3.0/lib/miscreant/aes/siv.rb:71:in `open'
	1: from /Users/andrew/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/miscreant-0.3.0/lib/miscreant/internals/aes/ctr.rb:31:in `encrypt'
/Users/user/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/miscreant-0.3.0/lib/miscreant/internals/util.rb:64:in `validate_bytestring': IV must be Encoding::BINARY (ArgumentError)