Fix for bad stacktraces in fennel.

fix.diff

diff --git a/fennel.lua b/fennel.lua
index 8c79320..58341df 100644
--- a/fennel.lua
+++ b/fennel.lua
@@ -2057,7 +2057,8 @@ local function traceback(msg, start)
     local level = start or 2 -- Can be used to skip some frames
     local lines = {}
     if msg then
-        table.insert(lines, msg)
+        local stripped = msg:gsub('^[^:]*:%d+:%s+', 'runtime error: ')
+        table.insert(lines, stripped)
     end
     table.insert(lines, 'stack traceback:')
     while true do