pw_env_setup: Add stderr error shell functions
pw_env_setup: Add stderr error shell functions

Add shell error functions that write to stderr.

Bug: b/227763851
Change-Id: Ifde7204b749d7985ede270189e1f1c837aad903d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/104941
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed