add system.onUnhandledException feature

lib/system/excpt.nim

@@ -230,6 +230,18 @@ when false:
     pushCurrentException(e)
     c_longjmp(excHandler.context, 1)
 
+var onUnhandledException*: (proc (errorMsg: string) {.
+  nimcall.}) ## set this error \
+  ## handler to override the existing behaviour on an unhandled exception.
+  ## The default is to write a stacktrace to ``stderr`` and then call ``quit(1)``.
+  ## Unstable API.
+
+template unhandled(buf, body) =
+  if onUnhandledException != nil:
+    onUnhandledException(buf)
+  else:
+    body
+
 proc raiseExceptionAux(e: ref Exception) =
   if localRaiseHook != nil:
     if not localRaiseHook(e): return
@@ -260,7 +272,9 @@ proc raiseExceptionAux(e: ref Exception) =
         add(buf, " [")
         add(buf, $e.name)
         add(buf, "]\n")
+        unhandled(buf):
           showErrorMessage(buf)
+          quitOrDebug()
       else:
         # ugly, but avoids heap allocations :-)
         template xadd(buf, s, slen: expr) =
@@ -276,6 +290,7 @@ proc raiseExceptionAux(e: ref Exception) =
         add(buf, " [")
         xadd(buf, e.name, e.name.len)
         add(buf, "]\n")
+        unhandled(buf):
           showErrorMessage(buf)
           quitOrDebug()