[FAB-7012] Improve error message for invalid identity

The bccsp MSP does not allow CA certificates to be
used as identities.  This change improves the error
message by better describing the issue.

Change-Id: Ie371d3fdb19bf5ff6a64d57efcf7b528764bb939
Signed-off-by: Gari Singh <gari.r.singh@gmail.com>

msp/mspimpl.go

@@ -442,7 +442,8 @@ func (msp *bccspmsp) getCertificationChainForBCCSPIdentity(id *identity) ([]*x50
 
 	// CAs cannot be directly used as identities..
 	if id.cert.IsCA {
-		return nil, errors.New("A CA certificate cannot be used directly by this MSP")
+		return nil, errors.New("An X509 certificate with Basic Constraint: " +
+			"Certificate Authority equals true cannot be used as an identity")
 	}
 
 	return msp.getValidationChain(id.cert, false)