transport: Move exception handler to the caller

transport: Move exception handler to the caller

Move exception handling code to the caller side. This is a preparation
for adding a custom error handler that is similar to UploadPack.

Note that `catch (Throwable t)` will not affect the exception spec since
Java will do an analysis of rethrown exceptions. See
https://docs.oracle.com/javase/8/docs/technotes/guides/language/catch-multiple.html.

Change-Id: I973a63d4b079c2cd1402afb3f2956e18b7d5a00c
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>