error reporting + flaky test

[matthid]

improve error reporting when we fail to retrieve information from a source, fix flaky nuspec extraction logic by removing the need to write to a temporary file.

Fixes the flaky test in #2454 (comment)

[matthid]

2) Error : Paket.IntegrationTests.RestoreSpec.#2496 Paket fails on projects that target multiple frameworks
System.Exception : Start of process dotnet failed. ApplicationName='dotnet', CommandLine='restore EmptyTarget.csproj', CurrentDirectory='/home/travis/build/fsprojects/Paket/integrationtests/scenarios/i002496-empty-target-framework/temp/EmptyTarget', Native error= Cannot find the specified file
  at Fake.ProcessHelper+ExecProcessWithLambdas@91-16.Invoke (System.String message) [0x00001] in <592836c5ccf1c534a7450383c5362859>:0 
  at Microsoft.FSharp.Core.PrintfImpl+StringPrintfEnv`1[TResult].Finalize () [0x00012] in <5893d081904cf4daa745038381d09358>:0 
  at Microsoft.FSharp.Core.PrintfImpl+Final2@235[TState,TResidue,TResult,A,B].Invoke (Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] env, A a, B b) [0x0005a] in <5893d081904cf4daa745038381d09358>:0 
  at Microsoft.FSharp.Core.OptimizedClosures+Invoke@3266-1[T2,T3,TResult,T1].Invoke (T2 u, T3 v) [0x00001] in <5893d081904cf4daa745038381d09358>:0 
  at Microsoft.FSharp.Core.OptimizedClosures+Invoke@3253[T2,TResult,T1].Invoke (T2 u) [0x00001] in <5893d081904cf4daa745038381d09358>:0 
  at Fake.ProcessHelper+ExecProcessWithLambdas@91-18.Invoke (System.String arg20) [0x00001] in <592836c5ccf1c534a7450383c5362859>:0 
  at Microsoft.FSharp.Core.FSharpFunc`2[T,TResult].InvokeFast[V] (Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] func, T arg1, TResult arg2) [0x0001f] in <5893d081904cf4daa745038381d09358>:0 
  at Fake.ProcessHelper.ExecProcessWithLambdas (Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] configProcessStartInfoF, System.TimeSpan timeOut, System.Boolean silent, Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] errorF, Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] messageF) [0x001c1] in <592836c5ccf1c534a7450383c5362859>:0 
  at Fake.ProcessHelper.ExecProcessAndReturnMessages (Microsoft.FSharp.Core.FSharpFunc`2[T,TResult] configProcessStartInfoF, System.TimeSpan timeOut) [0x0001c] in <592836c5ccf1c534a7450383c5362859>:0 
  at Paket.IntegrationTests.RestoreSpec.#2496 Paket fails on projects that target multiple frameworks () [0x000f4] in <596b790b91da6970a74503830b796b59>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00032] in <a07d6bf484a54da2861691df910339b1>:0 

@forki That one probably slipped through recently in #2526 and needs to be fixed for mono.

[matthid]

It really will take a long time until mono is in a stable state again. Mono 5.2.0 will probably break paket hard again if I look over the changes they want to include. Maybe we need to switch to dotnet-core for running tests on unix soon...

[forki]

thx!