No error nor warning is shown if a binding project contains a NativeReference to an inexistent framework

No error nor warning is shown if a binding project contains a NativeReference to a .framework directory that doesn't exist.

Test csproj:

```xml

Debug
AnyCPU
Library
NativeLibrary
NativeLibrary


bin\Debug


bin\Release







Framework

```

Create the two C# files:

```shell
touch ApiDefinition.cs
touch StructsAndEnums.cs
```

then build:

```shell
msbuild myproj.csproj

```

check the resources:

```shell
$ monodis --presources bin/Debug/NativeLibrary.dll

```

Just to show that an existing .framework works: change the `NativeReference` of the csproj to point to an actual framework on your system:

```xml

Framework

```

and then the framework is embedded:

```
$ msbuild
$ monodis --presources bin/Debug/NativeLibrary.dll
0: AppKit.framework (size 17954354)
```