chore(shellcheck): adopt die idiom for early termination in touched .sh files

Address kriskowal's CHANGES_REQUESTED review on #401: replace bare
`|| exit` after `cd` with `|| die "<context>"` so early-termination
sites emit a context line to stderr and exit non-zero, as with Perl's
`die`. Each touched script defines its own helper near the top:

    die() { printf '%s\n' "$*" >&2; exit 1; }

`printf` (not `echo -e`) avoids portability traps under /bin/sh.

Sites upgraded across the .sh files this PR touched:
- packages/nat/scripts/npm-audit-fix.sh: `cd nat`, `cd integration-test`
- scripts/npm-audit-fix.sh:              `cd ???`
- scripts/posttypedoc.sh:                `cd docs`, `cd api-docs`

scripts/check-packages.sh's terminal `exit "$EXIT"` and scripts/
shellcheck.sh's `exit 0` skip-when-empty are intentional orderly
exits, not the early-termination pattern the review objects to.
Left as-is.

The "throughout" scope is the .sh files this PR touched (the seven
plus the new scripts/shellcheck.sh); broader cross-tree adoption is
left to a separate dispatch if the maintainer wants it.

Author: endolinbot

packages/nat/scripts/npm-audit-fix.sh

@@ -1,6 +1,8 @@
 #!/bin/bash
+die() { printf '%s\n' "$*" >&2; exit 1; }
+
 git clone https://github.com/AgoricBot/nat.git
-cd nat || exit
+cd nat || die "cd: nat failed"
 git remote add upstream https://github.com/Agoric/nat.git
 git remote set-url origin https://AgoricBot:$GITHUB_TOKEN@github.com/AgoricBot/nat.git
 git fetch upstream
@@ -25,7 +27,7 @@ else
 fi
 
 # Do the same thing with the package.json in the integration-test folder
-cd integration-test || exit
+cd integration-test || die "cd: integration-test failed"
 if npm audit ; then
     echo "Nothing to fix"
 else

scripts/npm-audit-fix.sh

@@ -1,6 +1,8 @@
 #!/bin/bash
+die() { printf '%s\n' "$*" >&2; exit 1; }
+
 git clone https://github.com/AgoricBot/???.git
-cd ??? || exit
+cd ??? || die "cd: ??? failed"
 git remote add upstream https://github.com/Agoric/???.git
 git remote set-url origin https://AgoricBot:$GITHUB_TOKEN@github.com/AgoricBot/???.git
 git fetch upstream

scripts/posttypedoc.sh

@@ -1,9 +1,11 @@
 #!/bin/bash
+die() { printf '%s\n' "$*" >&2; exit 1; }
+
 (
-  cd docs || exit
+  cd docs || die "cd: docs failed"
   find images -type f -print0 | xargs -0 tar c
 ) |
 (
-  cd api-docs || exit
+  cd api-docs || die "cd: api-docs failed"
   tar xv
 )