[FAB-6386] Update error handling documentation
[FAB-6386] Update error handling documentation

This CR updates the error handling documentation to reflect the
(relatively) newly vendored github.com/pkg/errors, its usage, and
general guidelines for use within Fabric code.

Change-Id: I8f727e2489a267611fa50238d9a2d848335beac0
Signed-off-by: Will Lahti <f19a2779710675aad3df5c81e4ecfde11707e3aa@us.ibm.com>

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