With Python2.6, string exceptions are not allowed anymore. Turn them into ValueError

diff --git a/pyvnc2swf/rfb.py b/pyvnc2swf/rfb.py
index 1c99151..4fe49bc 100644
--- a/pyvnc2swf/rfb.py
+++ b/pyvnc2swf/rfb.py
@@ -603,7 +603,7 @@ class RFBFileParser(RFBProxy):
       try:
         length = {8:'B', 16:'H', 32:'L'}[bitsperpixel]
       except KeyError:
-        raise 'invalid bitsperpixel: %d' % bitsperpixel
+        raise ValueError('invalid bitsperpixel: %d' % bitsperpixel)
       unpackstr = endian + length
       nbytes = bitsperpixel / 8
       bits = {1:1, 3:2, 7:3, 15:4, 31:5, 63:6, 127:7, 255:8}
@@ -613,7 +613,7 @@ class RFBFileParser(RFBProxy):
              green_shift, green_max, 8-bits[green_max],
              blue_shift, blue_max, 8-bits[blue_max])
       except KeyError:
-        raise 'invalid {red,green,blue}_max: %d, %d or %d' % (red_max, green_max, blue_max)
+        raise ValueError('invalid {red,green,blue}_max: %d, %d or %d' % (red_max, green_max, blue_max))
       getrgb = eval(e)
       unpack_pixels = eval('lambda data: unpack("%s%%d%s" %% (len(data)/%d), data)' % (endian, length, nbytes))
       unpack_color1 = eval('lambda data: unpack("%s", data)' % unpackstr)