Revert removing error, only remove log

This preserves the existing error interface but it now prints using the buildpack output instead of the old `log` module.

CHANGELOG.md

@@ -12,7 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - `libherokubuildpack`:
-  - Removed `error` and `log` modules in favor of `buildpack_output`. ([#721](https://github.com/heroku/libcnb.rs/pull/721))
+  - Removed the `log` module in favor of `buildpack_output`. ([#721](https://github.com/heroku/libcnb.rs/pull/721))
 
 ### Added
 

libherokubuildpack/Cargo.toml

@@ -21,6 +21,7 @@ workspace = true
 default = ["command", "download", "digest", "tar", "toml", "fs", "write", "buildpack_output"]
 download = ["dep:ureq", "dep:thiserror"]
 digest = ["dep:sha2"]
+error = ["buildpack_output", "dep:libcnb"]
 tar = ["dep:tar", "dep:flate2"]
 toml = ["dep:toml"]
 fs = ["dep:pathdiff"]
@@ -40,7 +41,6 @@ libcnb = { workspace = true, optional = true }
 pathdiff = { version = "0.2.1", optional = true }
 sha2 = { version = "0.10.8", optional = true }
 tar = { version = "0.4.40", default-features = false, optional = true }
-termcolor = { version = "1.4.1", optional = true }
 thiserror = { version = "1.0.57", optional = true }
 toml = { workspace = true, optional = true }
 ureq = { version = "2.9.5", default-features = false, features = ["tls"], optional = true }

libherokubuildpack/src/error.rs

@@ -0,0 +1,71 @@
+use crate::buildpack_output::BuildpackOutput;
+use std::fmt::Debug;
+
+/// Handles a given [`libcnb::Error`] in a consistent style.
+///
+/// This function is intended to be used inside [`libcnb::Buildpack::on_error`].
+///
+/// It outputs generic libcnb errors in a consistent style using the [logging functions](log_error) from this
+/// crate. Buildpack specific errors are handled by the passed custom handler.
+///
+/// # Example:
+/// ```
+/// use libcnb::build::{BuildContext, BuildResult};
+/// use libcnb::Buildpack;
+/// use libcnb::detect::{DetectContext, DetectResult};
+/// use libcnb::generic::{GenericMetadata, GenericPlatform};
+/// use libherokubuildpack::log::log_error;
+/// use libherokubuildpack::error::on_error;
+///
+/// #[derive(Debug)]
+/// enum FooBuildpackError {
+///     CannotExecuteFooBuildTool(std::io::Error),
+///     InvalidFooDescriptorToml
+/// }
+///
+/// fn on_foo_buildpack_error(e: FooBuildpackError) {
+///     match e {
+///         FooBuildpackError::InvalidFooDescriptorToml => {
+///             log_error("Invalid foo.toml", "Your app's foo.toml is invalid!");
+///         }
+///         FooBuildpackError::CannotExecuteFooBuildTool(inner) => {
+///             log_error("Couldn't execute foo build tool", format!("Cause: {}", &inner));
+///         }
+///     }
+/// }
+///
+/// struct FooBuildpack;
+///
+/// impl Buildpack for FooBuildpack {
+///     type Platform = GenericPlatform;
+///     type Metadata = GenericMetadata;
+///     type Error = FooBuildpackError;
+///
+///     // Omitted detect and build implementations...
+///     # fn detect(&self, context: DetectContext<Self>) -> libcnb::Result<DetectResult, Self::Error> {
+///     #     unimplemented!()
+///     # }
+///     #
+///     # fn build(&self, context: BuildContext<Self>) -> libcnb::Result<BuildResult, Self::Error> {
+///     #     unimplemented!()
+///     # }
+///
+///     fn on_error(&self, error: libcnb::Error<Self::Error>) {
+///         on_error(on_foo_buildpack_error, error)
+///     }
+/// }
+/// ```
+pub fn on_error<F, E>(f: F, error: libcnb::Error<E>)
+where
+    E: Debug,
+    F: Fn(E),
+{
+    match error {
+        libcnb::Error::BuildpackError(buildpack_error) => f(buildpack_error),
+        libcnb_error => {
+            BuildpackOutput::new(std::io::stdout())
+                .start_silent()
+                .error(format!("Internal Buildpack Error\n\n{libcnb_error}"));
+        }
+    }
+}

libherokubuildpack/src/lib.rs

@@ -8,6 +8,8 @@ pub mod command;
 pub mod digest;
 #[cfg(feature = "download")]
 pub mod download;
+#[cfg(feature = "error")]
+pub mod error;
 #[cfg(feature = "fs")]
 pub mod fs;
 #[cfg(feature = "tar")]