Before, HardwareSerial1+.cpp were a copy of HardwareSerial1.cpp with all
0's replaced by the corresponding number. This would mean that e.g.
the Serial1 object would use the UBRRL register instead of UBRR1L when
it was defined, or the USART_RX_vect instead of USART1_RX_vect.

In practice, this would neve actually cause problems, since:
 - No avr chip currently has both the non-numbered registers as well as
   numbered registers.
 - HardwareSerial.h would only define HAVE_HWSERIALx when the
   corresponding numbered register is defined (except for
   HAVE_HWSERIAL0, which is also defined when the unnumbered registers
   are present).

Furthermore, before both the UARTx_xx_vect and USART_x_xx_vect was used.
Looking at the include files, only UART1_xx_vect is actually used (by
iom161.h), the others use USARTx_xx_vect. For this reason,
HardwareSerial1.cpp keeps the preprocessor conditional to select either
UART or USART and the other files use USART unconditionally.

While we're here, also fix the compiler error message when no valid ISR
name was found (it previously said "for the first UART" in all cases).