Don't clobber chip-tool error

[kpschoedel]

Problem

82cf1b1 (PR #9086) could clobber a returned error code with a
different value GetCommandExitStatus(), which could lead to chip-tool
exiting successfully when it shouldn't. (This doesn't actually happen
at present because nothing sets err after RunCommand(), but could
be triggered by otherwise innocuous code changes.)

Change overview

Check for an existing error in preference to GetCommandExitStatus().

Testing

Manual run of chip-tool with edits.

[github-actions]

Size increase report for "esp32-example-build" from 071592d

File	Section	File	VM
chip-temperature-measurement-app.elf	.flash.text	-36	-36
chip-shell.elf	.flash.text	48	48

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_info,0,1312
.debug_loc,0,1
.riscv.attributes,0,-2
.debug_abbrev,0,-9
.debug_line,0,-1482

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize
.debug_info,0,1336
[Unmapped],0,36
.debug_loc,0,16
.debug_str,0,-2
.flash.text,-36,-36
.debug_line,0,-1122

Comparing ./master_artifact/chip-ipv6only-app.elf and ./pull_artifact/chip-ipv6only-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-bridge-app.elf and ./pull_artifact/chip-bridge-app.elf:

sections,vmsize,filesize
.debug_info,0,1336
.debug_str,0,-2
.debug_line,0,-1182

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.debug_info,0,1336
.flash.text,48,48
[Unmapped],0,-48
.debug_line,0,-432

Comparing ./master_artifact/chip-lock-app.elf and ./pull_artifact/chip-lock-app.elf:

sections,vmsize,filesize
.debug_info,0,1312
.debug_abbrev,0,-9
.debug_loc,0,-15
.debug_line,0,-1152

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize