GC-safety error messages are useful now

tests/parallel/tgc_unsafe2.nim

@@ -0,0 +1,39 @@
+discard """
+  line: 28
+  nimout: '''tgc_unsafe2.nim(22, 5) Warning: 'trick' is not GC-safe as it accesses 'global' which is a global using GC'ed memory
+tgc_unsafe2.nim(26, 5) Warning: 'track' is not GC-safe as it calls 'trick'
+tgc_unsafe2.nim(28, 5) Error: 'consumer' is not GC-safe as it calls 'track'
+'''
+  errormsg: "'consumer' is not GC-safe as it calls 'track'"
+"""
+
+import threadpool
+
+type StringChannel = TChannel[string]
+var channels: array[1..3, StringChannel]
+
+type
+  MyObject[T] = object
+    x: T
+
+var global: MyObject[string]
+var globalB: MyObject[float]
+
+proc trick(ix: int) =
+  echo global.x
+  echo channels[ix].recv()
+
+proc track(ix: int) = trick(ix)
+
+proc consumer(ix: int) {.thread.} =
+  track(ix)
+
+proc main =
+  for ix in 1..3: channels[ix].open()
+  for ix in 1..3: spawn consumer(ix)
+  for ix in 1..3: channels[ix].send("test")
+  sync()
+  for ix in 1..3: channels[ix].close()
+
+when isMainModule:
+  main()