write_text has a couple of problems

[kbroman]

• font "8x6" seems to produce total garbage
• font "16x16" works only with capital letters. The warning message could be more informative. Currently something like In FUN(X[[i]], ...) : Character "h" not found

[kbroman]

• Fixed the font part.
• The warning message is maybe okay, but could be made more clear if we were more explicit: "Character [blah] not found in font [blah]".