better distinguish mmap errors & print errno

Sources/Backtrace/Backtrace.swift

@@ -53,10 +53,10 @@ private let fullCallback: CBacktraceFullCallback? = {
 }
 
 private let errorCallback: CBacktraceErrorCallback? = {
-    _, msg, _ in
+    _, msg, errNo in
     if let msg = msg {
-        _ = withVaList([msg]) { vaList in
-            vfprintf(stderr, "%s\n", vaList)
+        _ = withVaList([msg, errNo]) { vaList in
+            vfprintf(stderr, "SwiftBacktrace ERROR: %s (errno: %d)\n", vaList)
         }
     }
 }

Sources/CBacktrace/mmap.c

@@ -170,7 +170,7 @@ backtrace_alloc (struct backtrace_state *state,
       if (page == MAP_FAILED)
 	{
 	  if (error_callback)
-	    error_callback (data, "mmap", errno);
+	    error_callback (data, "mmap for alloc", errno);
 	}
       else
 	{

Sources/CBacktrace/mmapio.c

@@ -71,7 +71,7 @@ backtrace_get_view (struct backtrace_state *state ATTRIBUTE_UNUSED,
   map = mmap (NULL, size, PROT_READ, MAP_PRIVATE, descriptor, pageoff);
   if (map == MAP_FAILED)
     {
-      error_callback (data, "mmap", errno);
+      error_callback (data, "mmap file i/o", errno);
       return 0;
     }