don't raise exception in the default handler (#9783)

fixes #9657

lib/system/dyncalls.nim

@@ -17,13 +17,6 @@
 const
   NilLibHandle: LibHandle = nil
 
-proc c_fwrite(buf: pointer, size, n: csize, f: File): cint {.
-  importc: "fwrite", header: "<stdio.h>".}
-
-proc rawWrite(f: File, s: string|cstring) =
-  # we cannot throw an exception here!
-  discard c_fwrite(cstring(s), 1, s.len, f)
-
 proc nimLoadLibraryError(path: string) =
   # carefully written to avoid memory allocation:
   stderr.rawWrite("could not load: ")

lib/system/excpt.nim

@@ -17,8 +17,15 @@ var
     ## instead of stdmsg.write when printing stacktrace.
     ## Unstable API.
 
+proc c_fwrite(buf: pointer, size, n: csize, f: File): cint {.
+  importc: "fwrite", header: "<stdio.h>".}
+
+proc rawWrite(f: File, s: string|cstring) =
+  # we cannot throw an exception here!
+  discard c_fwrite(cstring(s), 1, s.len, f)
+
 when not defined(windows) or not defined(guiapp):
-  proc writeToStdErr(msg: cstring) = write(stdmsg, msg)
+  proc writeToStdErr(msg: cstring) = rawWrite(stdmsg, msg)
 
 else:
   proc MessageBoxA(hWnd: cint, lpText, lpCaption: cstring, uType: int): int32 {.

tests/exception/t9657.nim

@@ -0,0 +1,6 @@
+discard """
+  action: run
+  exitcode: 1
+"""
+close stdmsg
+writeLine stdmsg, "exception!"