FAKE error fails in MS.NET build but not Mono - mono

Question

I've got CI builds set up for my project that use MS .Net (with AppVeyor) and Mono (with Travis CI).
The current build has a problem in that it tries to run an executable which is missing. This fails the AppVeyor build:
Starting Target: Run Unit Tests (==> BuildUnitTests, BuildIntegrationTests, BuildSampleApplication)
Running build failed.
Error:
System.Exception: Start of process HttpClient.UnitTests/bin/Release\HttpClient.UnitTests.exe failed. The system cannot find the file specified
at Fake.ProcessHelper.directExec#218.Invoke(String message) in C:\code\fake\src\app\FakeLib\ProcessHelper.fs:line 218
at Fake.ProcessHelper.directExec(FSharpFunc`2 configProcessStartInfoF) in C:\code\fake\src\app\FakeLib\ProcessHelper.fs:line 213
at FSI_0001.Build.clo#68-3.Invoke(Unit _arg3) in C:\projects\http-fs\build.fsx:line 71
at Fake.TargetHelper.runSingleTarget(TargetTemplate`1 target) in C:\code\fake\src\app\FakeLib\TargetHelper.fs:line 411
But the Travis build just carries on regardless, apparently not getting an exception:
Starting Target: Run Unit Tests (==> BuildUnitTests, BuildIntegrationTests, BuildSampleApplication)
Cannot open assembly 'HttpClient.UnitTests/bin/Release/HttpClient.UnitTests.exe': No such file or directory.
Finished Target: Run Unit Tests
(see https://travis-ci.org/relentless/Http.fs/builds/57475713)
This is the code that fails (or not):
ProcessHelper.directExec (fun procInfo ->
procInfo.FileName <- Path.Combine(unitTestsDir, "bin/Release", "HttpClient.UnitTests.exe")
) |> ignore
Any idea why this difference exists, and more importantly how to make it the same on Mono? (In this situation, I'd really like the build to fail). Of course, in this case I could check for the file's existence, but ideally I want the build to fail on any problem.
Edit
The problem, specifically, is that the build does not fail under Mono when I think it should. It seems that the exception which is raised on MS .NET due to the missing exe is either not raised or swallowed on Mono, resulting in a build which passes when it should (IMO) fail.

Answer 1

Looks to me like the file ends up in bin/Debug, not bin/Release.
On line 1244 in the build output you have:
Copying file from '/home/travis/build/relentless/Http.fs/HttpClient.UnitTests/obj/Release/HttpClient.UnitTests.exe' to '/home/travis/build/relentless/Http.fs/HttpClient.UnitTests/bin/Debug/HttpClient.UnitTests.exe'