backtraces with --backtrace (#33478)

* backtraces without --debug

Currently `--debug` is too verbose and not-`--debug` gives to little
context about where exceptions are coming from.

So, instead, it'd be nice to have `spack --backtrace` and
`SPACK_BACKTRACE=1` as methods to get something inbetween: no verbose
debug messages, but always a full backtrace.

This is useful for CI, where we don't want to drown in debug messages
when installing deps, but we do want to get details where something goes
wrong if it goes wrong.

* completion

share/spack/spack-completion.bash

@@ -335,7 +335,7 @@ _spacktivate() {
 _spack() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -H --all-help --color -c --config -C --config-scope -d --debug --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -b --bootstrap -p --profile --sorted-profile --lines -v --verbose --stacktrace -V --version --print-shell-vars"
+        SPACK_COMPREPLY="-h --help -H --all-help --color -c --config -C --config-scope -d --debug --timestamp --pdb -e --env -D --env-dir -E --no-env --use-env-repo -k --insecure -l --enable-locks -L --disable-locks -m --mock -b --bootstrap -p --profile --sorted-profile --lines -v --verbose --stacktrace --backtrace -V --version --print-shell-vars"
     else
         SPACK_COMPREPLY="activate add arch audit blame bootstrap build-env buildcache cd change checksum ci clean clone commands compiler compilers concretize config containerize create deactivate debug dependencies dependents deprecate dev-build develop diff docs edit env extensions external fetch find gc gpg graph help info install license list load location log-parse maintainers make-installer mark mirror module patch pkg providers pydoc python reindex remove rm repo resource restage solve spec stage style tags test test-env tutorial undevelop uninstall unit-test unload url verify versions view"
     fi