By default, `Puppet::Util::Execution.execute` only returns stdout.
Puppet writes errors to stderr. When the run task fails, it only shows
the successful output:

```
$ bolt task run puppet_agent::run --targets puppet
Started on puppet...
Finished on puppet:
  Info: Using environment 'production'
  Info: Retrieving pluginfacts
  Info: Retrieving plugin
  Info: Loading facts
  Notice: Requesting catalog from puppet.local:8140 (10.0.1.1)
  Notice: Catalog compiled by puppet.local

  *long json here*
```

With the patch:

```
$ bolt task run puppet_agent::run --targets puppet
Started on puppet...
Finished on puppet:
  Info: Using environment 'production'
  Info: Retrieving pluginfacts
  Info: Retrieving plugin
  Info: Loading facts
  Notice: Requesting catalog from puppet.local:8140 (10.0.1.1)
  Notice: Catalog compiled by puppet.spirit-int.betadots.training
  Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, Failed to execute '/pdb/query/v4' on at least 1 of the following 'server_urls': https://puppetdb:8081 (file: /opt/puppetlabs/puppet/modules/infrastructure/manifests/puppet/puppetserver.pp, line: 11, column: 14) on node puppet.local
  Warning: Not using cache on failed catalog
  Error: Could not retrieve catalog; skipping run

  *long json here*
``

without this:

```
$ bolt plan run puppet_agent::run --targets puppet
Starting: plan puppet_agent::run
Starting: Check for Puppet agent on puppet
Finished: Check for Puppet agent with 0 failures in 2.75 sec
Starting: Run Puppet agent on puppet
Finished: Run Puppet agent with 0 failures in 17.45 sec
Finished: plan puppet_agent::run in 20.21 sec
Finished on puppet:
  Info: Using environment 'production'
  Info: Retrieving pluginfacts
  Info: Retrieving plugin
  Info: Loading facts
  Notice: Requesting catalog from puppet.local:8140 (10.0.1.1)
  Notice: Catalog compiled by puppet.local
  Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, Failed to execute '/pdb/query/v4' on at least 1 of the following 'server_urls': https://puppetdb:8081 (file: /opt/puppetlabs/puppet/modules/infrastructure/manifests/puppet/puppetserver.pp, line: 11, column: 14) on node puppet.local
  Warning: Not using cache on failed catalog
  Error: Could not retrieve catalog; skipping run
  *long json*
Successful on 1 target: puppet
Ran on 1 target
```

with this change:

```
$ bolt plan run puppet_agent::run --targets puppet
Starting: plan puppet_agent::run
Starting: Check for Puppet agent on puppet
Finished: Check for Puppet agent with 0 failures in 2.81 sec
Starting: Run Puppet agent on puppet
Finished: Run Puppet agent with 1 failure in 12.69 sec
Finished: plan puppet_agent::run in 15.5 sec
Failed on puppet:
  The task failed with exit code 1 and no stdout, but stderr contained:
  /opt/puppetlabs/puppet/lib/ruby/vendor_ruby/puppet/util/execution.rb:312:in `execute': Execution of '/opt/puppetlabs/bin/puppet agent -t --color false' returned 1: Info: Using environment 'production' (Puppet::ExecutionFailure)
  Info: Retrieving pluginfacts
  Info: Retrieving plugin
  Info: Loading facts
  Notice: Requesting catalog from puppet.local:8140 (10.0.1.1)
  Notice: Catalog compiled by puppet.local
  Error: Could not retrieve catalog from remote server: Error 500 on SERVER: Server Error: Evaluation Error: Error while evaluating a Function Call, Failed to execute '/pdb/query/v4' on at least 1 of the following 'server_urls': https://puppetdb:8081 (file: /opt/puppetlabs/puppet/modules/infrastructure/manifests/puppet/puppetserver.pp, line: 11, column: 14) on node puppet.local
  Warning: Not using cache on failed catalog
  Error: Could not retrieve catalog; skipping run
  	from /tmp/ee90dd59-55df-44a5-8b46-6d5b5f41445a/puppet_agent/tasks/run.rb:162:in `try_run'
  	from /tmp/ee90dd59-55df-44a5-8b46-6d5b5f41445a/puppet_agent/tasks/run.rb:187:in `run'
  	from /tmp/ee90dd59-55df-44a5-8b46-6d5b5f41445a/puppet_agent/tasks/run.rb:246:in `<main>'
Failed on 1 target: puppet
Ran on 1 target
```