It is actually not an error but a warning, that the certificate is self-issued and not issued by a trusted entity. A self-issued certificate offers the same encryption, but does not certify the identity of the website.
Certificates serve dual purposes: They encrypt the connection and they help ensuring the identity of the website. For certain types of websites it is important to know that the website is who it claims to be. For example for eCommerce where you are handing over money it is important to know the money goes where it should.
For control panels the reason for using a certificate is only to encrypt the connection, the identity of the website is of no significance. A commercial certificate is quite costly, requires a separate certificate for each server and must be reissued annually. Couple this with the all time low cost of hosting accounts I think you can understand why most hosting companies issue their own control panel certificates.
Most browsers let you to tick a box to avoid getting asked this question again, the one exception to this is Google Chrome, who for reasons only Google know does not offer this.