Exceptions instead of return codes, implement R

Now instead of returning codes, the methods of LPC raise exceptions with
the information the chip returned.

The new read_memory method implements Python bindings for the R command.

alpaca_isp/__init__.py

@@ -127,7 +127,9 @@ class LPC:
 
     def unlock(self, code="23130"):
         """Unlock the flash write, erase, and go commands"""
-        return self._send_command("U {}\r\n".format(code).encode("utf-8"))
+        r = self._send_command("U {}\r\n".format(code).encode("utf-8"))
+        if r[0] != ReturnCode.CMD_SUCCESS:
+            raise ISPError(r)
 
     @property
     def baudrate(self):
@@ -182,11 +184,23 @@ class LPC:
         # Ask to write data
         r = self._send_command("W {} {}\r\n".format(start, count).encode(
             "utf-8"))
+        if r[0] != ReturnCode.CMD_SUCCESS:
+            raise ISPError(r)
         # If the MCU is okay with what we intend to do, send the data
-        if r[0] == ReturnCode.CMD_SUCCESS:
-            # NOTE: this is right for LPC8xx chips, not others
-            ok = self._writeline(data[:count], plain=True)
-        return r
+        # NOTE: this is right for LPC8xx chips, not others
+        ok = self._writeline(data[:count], plain=True)
+        return
+
+    def read_memory(self, start, count):
+        """Read count bytes starting at the given address
+
+        Start and count must be multiples of four.
+        """
+        r = self._send_command("R {} {}\r\n".format(start, count).encode(
+            "utf-8"))
+        if r[0] != ReturnCode.CMD_SUCCESS:
+            raise ISPError(r)
+        return self._uart.read(count)
 
 
 class ReturnCode(Enum):