Have the per-capture-file-type open routines "wtap_open_offline()" calls

return 1 on success, -1 if they got an error, and 0 if the file isn't of
the type that file is checking for, and supply an error code if they
return -1; have "wtap_open_offline()" use that error code.  Also, have
the per-capture-file-type open routines treat errors accessing the file
as errors, and return -1, rather than just returning 0 so that we try
another file type.

Have the per-capture-file-type read routines "wtap_loop()" calls return
-1 and supply an error code on error (and not, as they did in some
cases, call "g_error()" and abort), and have "wtap_loop()", if the read
routine returned an error, return FALSE (and pass an error-code-pointer
argument onto the read routines, so they fill it in), and return TRUE on
success.

Add some new error codes for them to return.

Now that "wtap_loop()" can return a success/failure indication and an
error code, in "read_cap_file()" put up a message box if we get an error
reading the file, and return the error code.

Handle the additional errors we can get when opening a capture file.

If the attempt to open a capture file succeeds, but the attempt to read
it fails, don't treat that as a complete failure - we may have managed
to read some of the capture file, and we should display what we managed
to read.

svn path=/trunk/; revision=518

wiretap/ngsniffer.h

@@ -1,6 +1,6 @@
 /* ngsniffer.h
  *
- * $Id: ngsniffer.h,v 1.4 1998/12/17 06:39:13 gram Exp $
+ * $Id: ngsniffer.h,v 1.5 1999/08/19 05:52:55 guy Exp $
  *
  * Wiretap Library
  * Copyright (c) 1998 by Gilbert Ramirez <gram@verdict.uthscsa.edu>
@@ -21,5 +21,4 @@
  *
  */
 
-int ngsniffer_open(wtap *wth);
-int ngsniffer_read(wtap *wth);
+int ngsniffer_open(wtap *wth, int *err);