Fix bug #519922.

Fix bug #519922.

* AssemblyResolver.cs (FindInDirectory): Try the @reference as a
filename as it is, or with different @file_extensions.
If @reference is a assembly name, then look for *.@file_extensions
for target assembly.
(GetAssemblyNameFromFile): Log FileNotFoundException and BadImageFormatException
separately, instead of a single unclear message.

Change all uses of Assembly.GetAssemblyName with GetAssemblyNameFromFile
which does the required error checking and logging.

svn path=/trunk/mcs/; revision=137986