Python diagnostic messages truncated

I have written a python macro that uses the filtering/unwinding/decorator
facilities of gdb. 

(gdb) source /sna-stacktrace/stacktrace.py
(gdb) thr 3
[Switching to thread 3 (Thread 0x7f21b751e700 (LWP 22219))]
#0  0x00007f21ba364dad in write () at ../sysdeps/unix/syscall-template.S:81
81	../sysdeps/unix/syscall-template.S: No such file or directory.
(gdb) bt
#0  0x00007f21ba364dad in write () at ../sysdeps/unix/syscall-template.S:81
#1  0x0000000000614c78 in console_write (handle=<optimized out>,
buffer=0x7f21b8c06160, numbytes=1, byteswritten=0x7f21b751d814,
overlapped=<optimized out>) at io.c:1128
#2  0x000000000060d6b4 in ves_icall_System_IO_MonoIO
8-Write (handle=0x1,
src=0x7f21b8c06140, src_offset=<optimized out>, count=-1,
error=0x7f21b751d8d0) at file-io.c:862
#3  0x0000000041b90b35 in [wrapper -
managed-to-native]:System.IO.MonoIO.Write(long, szarray, int, int,
valuetype) ()
#4  0x0000000041b908e0 in System.IO.FileStream.WriteInternal(szarray, int,
int) ()
#5  0x0000000041b9073c in System.IO.FileStream.Write(szarray, int, int) ()
#6  0x0000000041b9063c in System.IO.StreamWriter.FlushBytes() ()
#7  0x0000000041b8e4f8 in System.IO.StreamWriter.FlushCore() ()
#8  0x0000000041b90fe0 in System.IO.StreamWriter.Write(szarray, int, int)
()
#9  0x0000000041b90cf8 in System.IO.CStreamWriter.Write(szarray, int, int)
()
#10 0x00007f21b751db10 in  ()
#11 0x0000000041b90c38 in System.IO.CStreamWriter.Write(szarray) ()
#12 0x0000000041b90bfb in System.IO.TextWriter.WriteLine() ()
#13 0x0000000041b8f9dd in System.IO.TextWriter.WriteLine(string) ()
#14 0x0000000041b8f97c in System.IO.SynchronizedWriter.WriteLine(string) ()
#15 0x00007f21b751dba0 in  ()
#16 0x0000000041b8883d in System.Console.WriteLine(string) ()
#17 0x0000000041b887f4 in Test.MainClass.Worker(object) ()
Python Exception <class 'gdb.error'> value has been optimized out:

It is that last message thats causing me grief. It appears there should
be more to the message but its not being shown and I am unable to find
out where the error is occurring. Is there a way of retrieving this
message?

Neale