Use return rather than exit in main().

Use return rather than exit in main().

Return 2 for a capture error - we mainly use 1 for command-line syntax
errors (rather than, say, filter syntax errors or an invalid interface).

Now that TShark exits with an error status when given an invalid capture
filter or invalid interface, check for "error" rather than "success" as
an exit status.

svn path=/trunk/; revision=33006