Relevant error information on plugin install may not be shown to user

When errors occur during the plugin installation, only the primary error is displayed (not the full ouput).

Sometime this may not be sufficient to diagnose an issue. The full output should be optionally displayed to provide better troubleshooting capability.

I have worked around this issue in the past by echo'ing the full output in bundler.rb [1]. However, due to the retry logic, this can get very ugly.

To reproduce, force a failure in the maven resolution by adding a junk proxy settings in the settings.xml like the one below [2].

[1]
```
diff --git a/lib/bootstrap/bundler.rb b/lib/bootstrap/bundler.rb
index ffb47e9cb..bf433d812 100644
--- a/lib/bootstrap/bundler.rb
+++ b/lib/bootstrap/bundler.rb
@@ -159,6 +159,9 @@ module LogStash
end
end
end
+ puts "**********************"
+ puts output
+ puts "**********************"
raise exception if exception

return output
```
[1]
```

local.dev.0
true
http
local.dev
9898
a
b


local.dev.1
true
https
local.dev
9898
c
d

```