fix(helm): handle errors when plugin command is not found
fix(helm): handle errors when plugin command is not found

If a 'command:' is not found for a plugin, it will not result in an
ExitError, but in a PathError. This prevents that condition from
panicing.

Closes #1609

SlickNik/helm,adamreese/deployment-manager,adamreese/deployment-manager,bradj/helm,technosophos/k8s-helm,kdada/helm,iamzhout/helm,FengyunPan/helm,FengyunPan/helm,michelleN/deployment-manager,appscode/helm,iamzhout/helm,fibonacci1729/helm,kdada/helm,michelleN/deployment-manager,helm/helm,rbwsam/helm,eicnix/helm,flynnduism/helm,technosophos/k8s-helm,eicnix/helm,technosophos/deployment-manager,adamreese/deployment-manager,SlickNik/helm,rbwsam/helm,helm/helm,sushilkm/helm,technosophos/deployment-manager,bacongobbler/helm,thomastaylor312/helm,adamreese/deployment-manager,jascott1/helm,adamreese/helm,michelleN/deployment-manager,qwangrepos/helm,sushilkm/helm,adamreese/deployment-manager,yan234280533/helm,adamreese/helm,qwangrepos/helm,technosophos/deployment-manager,technosophos/deployment-manager,appscode/helm,yan234280533/helm,technosophos/deployment-manager,michelleN/deployment-manager,flynnduism/helm,thomastaylor312/helm,bacongobbler/helm,bradj/helm,fibonacci1729/helm,jascott1/helm