feat(tvix/cli): add errors module with drv construction errors

These will be threaded through to eval through the new `TvixError`
variant.

Change-Id: Ia0d3f8710dcf26bb95015cd2a6a2b2911f06343f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7842
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/cli/src/errors.rs

@@ -0,0 +1,49 @@
+use std::{error, fmt::Display, rc::Rc};
+use tvix_derivation::DerivationError;
+
+#[derive(Debug)]
+pub enum Error {
+    // Errors related to derivation construction
+    DuplicateOutput(String),
+    ConflictingOutputTypes,
+    DuplicateEnvVar(String),
+    ShadowedOutput(String),
+    InvalidDerivation(DerivationError),
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Error::DuplicateOutput(name) => {
+                write!(f, "an output with the name '{name}' is already defined")
+            }
+
+            Error::ConflictingOutputTypes => write!(
+                f,
+                "fixed-output derivations can only have the default `out`-output"
+            ),
+
+            Error::DuplicateEnvVar(name) => write!(
+                f,
+                "the environment variable '{name}' has already been set in this derivation"
+            ),
+            Error::ShadowedOutput(name) => write!(
+                f,
+                "the environment variable '{name}' shadows the name of an output"
+            ),
+            Error::InvalidDerivation(error) => write!(f, "invalid derivation parameters: {error}"),
+        }
+    }
+}
+
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        None
+    }
+}
+
+impl From<Error> for tvix_eval::ErrorKind {
+    fn from(err: Error) -> Self {
+        tvix_eval::ErrorKind::TvixError(Rc::new(err))
+    }
+}