armv8: log the register name which we failed to read or write

armv8: log the register name which we failed to read or write

when openocd fails to read armv8 register, the user is not informed
which register has caused the error.

for example, in AArch32 state ESR_EL3 read/write is not supported,
thus armv8_dpm_read_current_registers is always failing without mentioning
which register has caused the error.

Change-Id: I24c5abbda9fac24fb77a01777ed15261aeaaf800
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5516
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>