[Fix #1187] Only write to STDERR if an error happened

[0x53A]

Fixes #1187

Before: If the build was successfull, it would still write an empty line to STDERR, causing issues with powershell.

Now: it only writes if the error message is not empty.

[forki]

Ah very good!

[forki]

Thanks for taking care of this.

[0x53A]

Great, thank you! I see there is already a new nuget, so I can test it on monday at work.
I absolutely love CI and continuous deployment!

[matthid]

I'm not sure this is the correct fix (don't get me wrong I think the patch is a good idea anyway). My understanding of the code-base is that traceFAKE tries to write into the std-err stream but only in environments which will not cause the build to fail (otherwise it will write to regular std-out).

The logic for this is https://github.com/fsharp/FAKE/blob/master/src/app/FakeLib/TraceListener.fs#L87
So while you fixed one traceFAKE call you might run into the same trouble again in the future in another place where traceFAKE is used. So the question is can we detect if we are in an $ErrorActionPreference = true environment?

Just one note traceFAKE is not used that much throughout the codebase but traceImportant has the same behavior as it is defined almost the same way (https://github.com/fsharp/FAKE/blob/master/src/app/FakeLib/TraceHelper.fs#L49) and is used on quite some places.

Furthermore, I consider that PowerShell must be destroyed.