curated package predated the standard errors package introduced in
go1.13
the standard package does a better job of what curated attempted to do
the change of package also gave me a opportunity to clean up the error
messages a little bit
removed PCZERO target, which is too specific to be useful. NONEXE halts
execution when the PC reaches any address that isn't a Cartridge or RAM
address.
I think originally, after the debugging loop refactor, I was planning on
implementing this as STEP OVER and allowing a regular STEP (even in
instruction quantum) to advance to the next clock. but it proved to be
annoying and confusing
two reasons:
1) to keep performance acceptable playmode only checks halting
conditions on a CPU instruction boundary. however a CLK changes many
times during an instruction meaning it will never match.
2) a CLK breakpoint will always match within 228 emulation ticks so
there is no point entering playmode at all because it will definitely
drop back to the debugger (within microseconds)
added a range change check to SCANLINE and CLK targets in
breakpoints.parseCommand(). we know what the possible values are for
these targets so we can be helpful and inform the user the some values
will never match
Instruction boundaries can be inferred by the debugging input loop,
which is the only place it is requred. reworked GetAdjustedCoords() in
the televsion package (now called AdjCoords()) to cope with this.
ReqState() is totally replaced by GetCoords()
halting conditions are checked every video-cycle. this reverts a
temporary change made in a73dcae6dcc61e4ced03f23fe4df504cb966fb36
renamed files in debugger package for clarity
