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

libraries/SoftwareSerial/SoftwareSerial.h

@@ -89,7 +89,7 @@ public:
   bool overflow() { bool ret = _buffer_overflow; _buffer_overflow = false; return ret; }
   int peek();
 
-  virtual ssize_t write(uint8_t byte);
+  virtual size_t write(uint8_t byte);
   virtual int read();
   virtual int available();
   virtual void flush();