toCLIError() panics on unhandled error types (e.g. helmexec.ExitError
from a failed helm plugin install). On Windows, plugin install hooks
often fail due to missing 'sh', causing helmfile init to crash even
when the plugin binary was placed correctly.
- Add helmexec.ExitError case to toCLIError and replace panic in the
default case with a graceful error return
- After AddPlugin/UpdatePlugin errors, verify whether the plugin is
actually present before failing; log a warning and continue if so
Fixes#1983
Signed-off-by: Aditya Menon <amenon@canarytechnologies.com>