Better output

[giuliomoro]

1. error messages go to stderr
2. they are prefixed with pasm: (admittedly not useful to everyone)
3. when macro expansion fails you get a copy of the line where it fails, making debug much easier, given that the meaning of line numbers is often confusing.
   This turns, e.g. a cryptic message like:

spi-pru.p(588) Error: Operand 4 immediate value out of range (1-124)
spi-pru.p(588) Error: (While expanding code line 4 of macro 'BUS_MODE_MASTER_TRANSMITTER')
spi-pru.p(588) Error: (While expanding code line 16 of macro 'START_SCAN')
spi-pru.p(588) Error: (While expanding code line 15 of macro 'MASTER_MODE')
spi-pru.p(617) Error: Operand 4 immediate value out of range (1-124)
spi-pru.p(617) Error: (While expanding code line 5 of macro 'BUS_MODE_MASTER_RECEIVER')
spi-pru.p(620) Error: Operand 4 immediate value out of range (1-124)
spi-pru.p(620) Error: (While expanding code line 4 of macro 'BUS_MODE_MASTER_TRANSMITTER')

into a readable one:

spi-pru.p(588) Error: Operand 4 immediate value out of range (1-124)
spi-pru.p(588) Error: (While expanding code line 4 of macro 'BUS_MODE_MASTER_TRANSMITTER': ``LBBO r6, r26, r5, SPICH0_WL_BYTES'')
spi-pru.p(588) Error: (While expanding code line 16 of macro 'START_SCAN': ``BUS_MODE_MASTER_TRANSMITTER r25, r26, r21'')
spi-pru.p(588) Error: (While expanding code line 15 of macro 'MASTER_MODE': ``START_SCAN'')
spi-pru.p(617) Error: Operand 4 immediate value out of range (1-124)
spi-pru.p(617) Error: (While expanding code line 5 of macro 'BUS_MODE_MASTER_RECEIVER': ``LBBO r6, r2, r5, SPICH0_WL_BYTES'')
spi-pru.p(620) Error: Operand 4 immediate value out of range (1-124)
spi-pru.p(620) Error: (While expanding code line 4 of macro 'BUS_MODE_MASTER_TRANSMITTER': ``LBBO r6, r2, r5, SPICH0_WL_BYTES'')