fix: improve pnpm installation script

[Tguntenaar]

Problem

When building a custom image via frappe_docker with the gameplan app included in
apps.json, bench init fails during asset building with a confusing
exit code 127 from Yarn.

The root cause: install-pnpm.sh unconditionally exits 0 even when
npm install -g pnpm@latest-10 silently fails (e.g. in minimal Docker
images where global installs can behave unexpectedly). The build then
continues into pnpm install && pnpm build with no pnpm binary
available, which blows up deep in the Yarn/bench build chain.

The log looks like this:

pnpm could not be found, installing...
pnpm version:
...
status: 127
error Command failed with exit code 1.

Fix

• Add set -euo pipefail so failures aren't silently swallowed.
• After the install attempt, verify pnpm is actually on PATH before
  continuing. If it's not, exit 1 with a clear message instead of
  letting the build proceed and fail later with a misleading error.
• Remove the unconditional exit 0 at the end.