Remove "Error:" prefix from error message

We had already fixed the others, but this one was overlooked.
diff --git a/vncviewer/FLTKPixelBuffer.cxx b/vncviewer/FLTKPixelBuffer.cxx
index 1eafe10..ab116d1 100644
--- a/vncviewer/FLTKPixelBuffer.cxx
+++ b/vncviewer/FLTKPixelBuffer.cxx
@@ -30,7 +30,7 @@
 {
   data = new rdr::U8[width * height * format.bpp/8];
   if (data == NULL)
-    throw rfb::Exception(_("Error: Not enough memory for framebuffer"));
+    throw rfb::Exception(_("Not enough memory for framebuffer"));
 }
 
 FLTKPixelBuffer::~FLTKPixelBuffer()