Check that yarn is installed when updating
Check that yarn is installed when updating

If yarn isn't installed the user gets a vague "yarn failed in #{plugin}"
error which isn't obvious what failed.

We can check that the command exists prior to running yarn and thus
print a helpful error message.

ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic,ManageIQ/manageiq-ui-classic