* corlib_test.dll.sources: Added FileNotFoundExceptionTest.cs.
* Exception.cs: Marked message internal to allow derived classes to
access the raw message (without changing the public API).
* FileNotFoundException.cs: Changed message for default ctor to match
MS. Use internal message field of Exception to check whether Message
is null. On 2.0 profile, use file/assembly load failure message when
no message is set and a filename was specified. On 1.0 profile,
always use file/assembly load failure message when no message is set
(regardless of whether a filename was specified or not). Made some
cosmetic changes to ToString to have it match MS.
* FileNotFoundExceptionTest.cs: Added ctor tests.
svn path=/trunk/mcs/; revision=65792