logging: Log the cause behind agent startup failure (#11353)

Log the failure error when the agent fails to start. Previously, the
agent startup failure error would be emitted to the command UI but not
logged. So it doesn't get emitted to syslog or `log_file` if they are
set, and it makes debugging much harder. Also, logging the error again
before exit makes the error more visible: previously, the operator
needed to scroll to the top to find the error.

On a sample failure, the output will look like:
```
==> WARNING: Bootstrap mode enabled! Potentially unsafe operation.
==> Loaded configuration from sample-configs/config-bad
==> Starting Nomad agent...
==> Error starting agent: setting up server node ID failed: mkdir /path-without-permission: read-only file system
    2021-10-20T14:38:51.179-0400 [WARN]  agent.plugin_loader: skipping external plugins since plugin_dir doesn't exist: plugin_dir=/path-without-permission/plugins
    2021-10-20T14:38:51.181-0400 [DEBUG] agent.plugin_loader.docker: using client connection initialized from environment: plugin_dir=/path-without-permission/plugins
    2021-10-20T14:38:51.181-0400 [DEBUG] agent.plugin_loader.docker: using client connection initialized from environment: plugin_dir=/path-without-permission/plugins
    2021-10-20T14:38:51.181-0400 [INFO]  agent: detected plugin: name=java type=driver plugin_version=0.1.0
    2021-10-20T14:38:51.181-0400 [INFO]  agent: detected plugin: name=docker type=driver plugin_version=0.1.0
    2021-10-20T14:38:51.181-0400 [INFO]  agent: detected plugin: name=mock_driver type=driver plugin_version=0.1.0
    2021-10-20T14:38:51.181-0400 [INFO]  agent: detected plugin: name=raw_exec type=driver plugin_version=0.1.0
    2021-10-20T14:38:51.181-0400 [INFO]  agent: detected plugin: name=exec type=driver plugin_version=0.1.0
    2021-10-20T14:38:51.181-0400 [INFO]  agent: detected plugin: name=qemu type=driver plugin_version=0.1.0
    2021-10-20T14:38:51.181-0400 [ERROR] agent: error starting agent: error="setting up server node ID failed: mkdir /path-without-permission: read-only file system"
```

This change adds the final `ERROR` message. It's easy to miss the `==>
Error starting agent` above.

.changelog/11353.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+agent: Log the cause of failure if agent failed to start
+```

command/agent/command.go

@@ -492,6 +492,8 @@ func (c *Command) setupAgent(config *Config, logger hclog.InterceptLogger, logOu
 	c.Ui.Output("Starting Nomad agent...")
 	agent, err := NewAgent(config, logger, logOutput, inmem)
 	if err != nil {
+		// log the error as well, so it appears at the end
+		logger.Error("error starting agent", "error", err)
 		c.Ui.Error(fmt.Sprintf("Error starting agent: %s", err))
 		return err
 	}