Add `StringErrorPlaceholder` error variant

Allows some context to be passed in placeholder errors.  Like
.exepect() instead of .unwrap() (GenericErrorPlaceholder).

Change-Id: I3d4027560b55a6b8879db62402a5f752e6c520c4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126255
Pigweed-Auto-Submit: Erik Gilling <konkers@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
1 file changed
tree: 4eb433fe52abfe6f5037cee0dd16d42e42f45fd8
  1. examples/
  2. qg/
  3. qg-cli/
  4. tools/
  5. .gitignore
  6. Cargo.toml
  7. rustfmt.toml