Better handling of attempts to load PKCS#11 library with incorrect bitness

[jariq]

Pkcs11Interop 3.1 throws exception with hard to understand message "Unable to load library: %1 is not a valid Win32 application" when there is an attempt to load 32-bit PKCS#11 library in 64-bit runtime or 64-bit PKCS#11 library in 32-bit runtime. This problem seems to be hit quite frequently (e.g. in #25).

Code generating this message was removed in 4e45617 as a side effect of adding support for netstandard1.1 compatible platforms so this message should never appear in the exception again but I still believe such frequent error deserves its own exception type or at least exception message.

ERROR_BAD_EXE_FORMAT error code returned by Marshal.GetLastWin32Error() seems to be a natural choice for detection whether there was an attempt to load PKCS#11 library with incorrect bitness but I am not sure whether the same error code is returned on all supported platforms so I will need to investigate this further and "manually" check the code (or find other solution) on all platforms.

Following table tracks my progress:

Platform	Runtime	Solution
Windows 10	net20 32-bit	GetLastWin32Error() returns 0xC1
Windows 10	net20 64-bit	GetLastWin32Error() returns 0xC1
Windows 10	net40 32-bit	GetLastWin32Error() returns 0xC1
Windows 10	net40 64-bit	GetLastWin32Error() returns 0xC1
Windows 10	sl5 32-bit	I can't get it working on 64-bit OS
Windows 10	sl5 64-bit	GetLastWin32Error() returns 0xC1
Windows 10	corefx 32-bit	GetLastWin32Error() returns 0xC1
Windows 10	corefx 64-bit	GetLastWin32Error() returns 0xC1
Linux	mono/corefx	5th byte of ELF binary provides architecture info
Mac OS X	mono/corefx	dlopen_preflight checks if the library architecture is compatible with the current process
Android	Xamarin	5th byte of ELF binary provides architecture info
iOS	Xamarin	PKCS#11 library is linked statically so it cannot be targeting invalid platform