i2c master exceptions now use indicate_error from picerror.h
added define I2C_MASTER_RECVLEN for the maximum receive length

temporary measure until I can decide how to indicate the maximum bytes
to read within the i2c request.  (limited by message size)

this combats an issue with the current i2c slave code where it holds the
bus until a stop or restart is asserted.
added codes to picerror.h
added a comment to sample.c
added missing reset in messageframe when a message is released
updated some sample rates and slave address in sample.h - all just testing
added extra condition for relaying I2C messages received over UART