feat: log failure to add plugin in "plugin test"

[tbobm]

Summary

Display the internal reason why the plugin failed to be installed.

Error are currently returned but unused and there are several reasons why the plugin addition could fail

Current:

$ go run cmd/asdf/main.go plugin test my-plugin some-url install 1.1.1
FAILED: my-plugin was not properly installed
exit status 1

With changes:

$ go run cmd/asdf/main.go plugin test my-plugin some-url install 1.1.1
FAILED: my-plugin was not properly installed reason: Plugin named asdf-test-my-plugin already added
exit status 1