Report an error message when dependent libraries can't be found

[rpavlik]

While the DLL search path fixer should keep this from happening, Managed-OSVR shouldn't silently fail if it can't load the DLLs it's trying to p/invoke, which is most difficult when it's a dependency of the invoked DLL that's missing.

On a related note, if the directly-invoked DLLs are missing (such as when trying to run a 64-bit Unity build when there's no 64-bit Unity plugin yet), that should also be a visible error condition.

[rpavlik]

Note that it looks like there is an attribute (DefaultDllImportSearchPathsAttribute) that (at least in non-Mono newer .NET runtimes) helps with search paths.

References:

• https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportsearchpath(v=vs.110).aspx
• https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.defaultdllimportsearchpathsattribute(v=vs.110).aspx
• https://bugzilla.xamarin.com/show_bug.cgi?id=27494

Building multiple version using build configurations might be useful:

• http://stackoverflow.com/questions/2923210/conditional-compilation-and-framework-targets