Clean up and prepare FileLoadException.cs for move to Shared Partition. (#10764)

Clean up and prepare FileLoadException.cs for move to Shared Partition. (#10764)

* String => string

* Replace fields with autoprops.

* Turn Type.FullName call to Type.ToString() in ToString().

Avoids MissingMetadataExceptions on CoreRT.

Assuming metadata exists, ToString() and FullName()
still generates the same string except in the weird
case where an app subclasses FileLoadException
with a generic exception type - in which case,
the ToString() version is actually more readable.

* Get rid of useless try-catch around FusionLog calls.

FusionLog is now just a normal property and an answer
to a trivia question - it won't throw SecurityException.

* Get rid of low-value SetMessageField() middleman.

* Split off the stuff we don't want to share.

* The complementary carwash...

* Don't conditionalize FusionLog serialization.