Allow to get the String representation of an error number

Motivation:

At the moment it is only possible to get the String representation of the last error by calling SSL.getLastError() which will allocate a String to return it.
Often it is cheaper to just call SSL.getLastErrorNumber() as it only returns a long and then only create the String representation of the error number if really necessary via an extra method call.

Modifications:

Add SSL.getErrorString(long) which allows to retrieve a String representation of a error number / code.

Result:

More efficient error handling possible.