Raising strings is deprecated in Python 2.5, raise an exception instead.

git-svn-id: svn://svn.code.sf.net/p/fonttools/code/trunk@569 4cde692c-a291-49d1-8350-778aa11640f8
diff --git a/Lib/fontTools/ttLib/tables/ttProgram.py b/Lib/fontTools/ttLib/tables/ttProgram.py
index 2683669..ec828a5 100644
--- a/Lib/fontTools/ttLib/tables/ttProgram.py
+++ b/Lib/fontTools/ttLib/tables/ttProgram.py
@@ -173,7 +173,11 @@
 streamOpcodeDict, streamMnemonicDict = _makeDict(streamInstructions)
 opcodeDict, mnemonicDict = _makeDict(instructions)
 
-tt_instructions_error = "TT instructions error"
+class tt_instructions_error(Exception):
+	def __init__(self, error):
+		self.error = error
+	def __str__(self):
+		return "TT instructions error: %s" % repr(self.error)
 
 
 _comment = r"/\*.*?\*/"