feat(cli): write any errors to stderr, fail if requested

The command line program provided by the crate by default succeeds,
despite any errors. If PANDOC_FILTER_FAIL=1 is set in the environment,
it fails.

Sponsored-by: author

1 files changed, 27 insertions, 1 deletions

diff --git a/pandoc-filter-diagram.md b/pandoc-filter-diagram.md
index 12520cb..30e3cf5 100644
--- a/pandoc-filter-diagram.md
+++ b/pandoc-filter-diagram.md
@@ -123,7 +123,7 @@ the HTML output, not referenced as an external image.
 ~~~scenario
 given an installed Rust program pandoc-filter-diagram
 given file plantuml.md
-when I run pandoc --filter subplot-filter plantuml.md -o plantuml.html
+when I run pandoc --filter pandoc-filter-diagram plantuml.md -o plantuml.html
 then file plantuml.html matches regex /img src="data:image/svg\+xml;base64,/
 ~~~
 
@@ -252,3 +252,29 @@ blocked:
   - next
 ~~~
 ~~~~~~~~
+
+
+# Command line tool
+
+The crate provides a command line tool `pandoc-filter-diagram`, for
+use with the `pandoc --filter` option. By default, it ignores any
+errors, but if the `PANDOC_FILTER_FAIL` environment variable is set
+to 1, it fails if there were any errors. This scenario verifies that
+this happens. The happy cases are already verified by other scenarios,
+so this one only verifies handling of bad input.
+
+~~~scenario
+given an installed Rust program pandoc-filter-diagram
+given file bad.md
+when I run pandoc --filter pandoc-filter-diagram bad.md -o bad.html
+then exit code is 0
+when I try to run env PANDOC_FILTER_FAIL=1 pandoc --filter pandoc-filter-diagram bad.md -o bad.html
+then command fails
+~~~
+
+~~~{#bad.md .file .markdown .numberLines}
+trigraph "example" {
+thing -> other
+}
+~~~
+