Capture stderr output of commands executed by Command.exec()

[youpong]

The Problem

In the current implementation, only the standard output is captured because the command is executed by popen(3).
Implement it so that it also captures standard error output.

Related Issue

• Handle failures and exceptions #148 - Need to capture stderr in case of failure to get theme.

[TanmayPatil105]

@youpong Is this in progress? I was thinking of making a release soon!

[youpong]

It's in progress. It will probably take a couple of weeks.
It's hard to estimate exactly how much work there is because it involves a technical first for me.