[FAB-7012] Improve error message for invalid identity
[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 <10008a422f4be611b73174084ba2f7f909d5b402@gmail.com>

lukehuangch/fabric,muralisrini/fabric,ckeyer/fabric,ckeyer/fabric,ckeyer/fabric,binhn/fabric,binhn/fabric,stemlending/fabric,lukehuangch/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,ckeyer/fabric,xixuejia/fabric,lukehuangch/fabric,ckeyer/fabric,hyperledger/fabric,manish-sethi/fabric-sidedb,muralisrini/fabric,manish-sethi/fabric-sidedb,xixuejia/fabric,lukehuangch/fabric,lukehuangch/fabric,manish-sethi/fabric,stemlending/fabric,ckeyer/fabric,stemlending/fabric,lukehuangch/fabric,binhn/fabric,hyperledger/fabric,lukehuangch/fabric,manish-sethi/fabric,xixuejia/fabric,stemlending/fabric,jimthematrix/fabric,muralisrini/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,jimthematrix/fabric,ckeyer/fabric