Moving write errors out of return value into separate API methods.

write(), print(), println() now return size_t (and don't use negative values to signal errors).
Print adds writeError() for checking for write errors, clearWriteError() to reset the flag to false, and a protected setWriteError() for signalling errors.

http://code.google.com/p/arduino/issues/detail?id=598

LiquidCrystal.cpp

@@ -258,7 +258,7 @@ inline void LiquidCrystal::command(uint8_t value) {
   send(value, LOW);
 }
 
-inline ssize_t LiquidCrystal::write(uint8_t value) {
+inline size_t LiquidCrystal::write(uint8_t value) {
   send(value, HIGH);
   return 1; // assume sucess
 }