add check for return value of FT_Glyph_To_Bitmap()

[the-shank]

FT_Glyph_To_Bitmap() can fail (for example, due to a failed memory allocation in ft_new_glyph() that is called by FT_Glyph_To_Bitmap()) and would return a non-zero value to signal an error. This patch adds a check to the return value of FT_Glyph_To_Bitmap() to check for non-zero return value and throw an exception in case of error.