UI message with key headers doesn't encompass S/MIME options

[rrrooommmaaa]

In KeyImportUI.rejectIfNot there is a message saying something like:
This was a public key. Please insert a private key instead. It's a block of text starting with -----BEGIN PGP PRIVATE KEY BLOCK-----
or
This was a private key. Please insert a public key instead. It's a block of text starting with -----BEGIN PGP PUBLIC KEY BLOCK-----.

No S/MIME options like
-----BEGIN PRIVATE KEY-----
-----BEGIN RSA PRIVATE KEY-----
-----BEGIN CERTIFICATE-----

or possibly (encrypted)
-----BEGIN ENCRYPTED PRIVATE KEY-----
-----BEGIN PKCS12 FILE-----
are specified there.

[tomholub]

We could remove the It's a block of text starting with .... instead of listing them all.