Strings for IO driver errors

lib/base.tri

@@ -74,7 +74,7 @@ succ = y (self :
       t))
 
 ok = value rest : pair true (pair value rest)
-err = code rest : pair false (pair code rest)
+err = msg rest : pair false (pair msg rest)
 
 matchResult = (errCase okCase result :
   matchPair

lib/io.tri

@@ -63,24 +63,24 @@ onWriteFile = (path contents errCase okCase :
 
 readFileOrPrintError = (path okCase :
   onReadFile path
-    (err rest : putStrLn "Read failed")
+    (err rest : putStrLn (append "Read failed: " err))
     okCase)
 
 writeFileOrPrintError = (path contents okCase :
   onWriteFile path contents
-    (err rest : putStrLn "Write failed")
+    (err rest : putStrLn (append "Write failed: " err))
     okCase)
 
 copyFile = (src dst :
   bind (readFile src)
     (result :
       matchResult
-        (err rest : putStrLn "Read failed")
+        (err rest : putStrLn (append "Read failed: " err))
         (contents rest :
           bind (writeFile dst contents)
             (wr :
               matchResult
-                (err rest : putStrLn "Write failed")
+                (err rest : putStrLn (append "Write failed: " err))
                 (ok rest   : pure t)
                 wr))
         result))