Improve dotnet tool output

[MarcoRossignoli]

Working on #513 found not so clear where is the problem in case of missing startup process file name.
This PR will improve error message

Excluded module filter '[xunit*]*'
Instrumented module: 'C:\git\marcorossignoli.github.io\src\coverlet\Issue0\CoverletTest\bin\Debug\netcoreapp2.1\CoverletSampleLib.dll'
Excluded module: 'C:\git\marcorossignoli.github.io\src\coverlet\Issue0\CoverletTest\bin\Debug\netcoreapp2.1\xunit.runner.reporters.netcoreapp10.dll'
Excluded module: 'C:\git\marcorossignoli.github.io\src\coverlet\Issue0\CoverletTest\bin\Debug\netcoreapp2.1\xunit.runner.utility.netcoreapp10.dll'
Excluded module: 'C:\git\marcorossignoli.github.io\src\coverlet\Issue0\CoverletTest\bin\Debug\netcoreapp2.1\xunit.runner.visualstudio.dotnetcore.testadapter.dll'
Start process 'vstest.console' failed with 'The system cannot find the file specified'

cc: @tonerdo @petli @Nusserdt