Use error output as TaskExitException message

[prudloff-insite]

Overview

This pull request:

• Fixes a bug
• Adds a feature
• Breaks backwards compatibility
• Has tests that cover changes
• Adds or fixes documentation

Summary

Fixes #1153.

Description

TaskExitException contains STDOUT, which causes the stand output to be displayed twice and the actual error message to be buried between it.
Making the exception contain STDERR ensure the error message is the last thing printed, so the can find it easily. (It also makes more sense semantically that an exit exception contains the error output.)

With the patch

 [ExecStack] echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && invalid_cmd
 [ExecStack] Running echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 invalid_cmd
this command output is very long
this command output is very long
this command output is very long
this command output is very long
this command output is very long
this command output is very long
sh: 1: invalid_cmd: not found
 [ExecStack]  Exit code 127  Time 0.003s
 [notice] Stopping on fail. Exiting....
 [error]  Exit Code: 127 
 [error]    in task Robo\Task\Base\ExecStack 

  sh: 1: invalid_cmd: not found
 

Without the patch

 [ExecStack] echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && echo "this command output is very long" && invalid_cmd
 [ExecStack] Running echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 echo "this command output is very long" &&
 invalid_cmd
this command output is very long
this command output is very long
this command output is very long
this command output is very long
this command output is very long
this command output is very long
sh: 1: invalid_cmd: not found
 [ExecStack]  Exit code 127  Time 0.003s
 [notice] Stopping on fail. Exiting....
 [error]  Exit Code: 127 
 [error]    in task Robo\Task\Base\ExecStack 

  this command output is very long
this command output is very long
this command output is very long
this command output is very long
this command output is very long
this command output is very long
 

[prudloff-insite]

@greg-1-anderson Would you be able to review this?