have prepare add a generic PrepareError context

if a more specifc one hasn't already been provided

this way we can easily distinguish BuildDirectory::run failing due to Prepare::prepare failing from it failing for other reasons

Author: Skgland

src/build.rs

@@ -1,6 +1,6 @@
 use crate::cmd::{Command, MountKind, Runnable, SandboxBuilder};
 use crate::prepare::Prepare;
-use crate::{Crate, Toolchain, Workspace};
+use crate::{Crate, PrepareError, Toolchain, Workspace};
 use std::path::PathBuf;
 use std::vec::Vec;
 
@@ -194,7 +194,13 @@ impl BuildDirectory {
         }
 
         let mut prepare = Prepare::new(&self.workspace, toolchain, krate, &source_dir, patches);
-        prepare.prepare()?;
+        prepare.prepare().map_err(|err| {
+            if err.downcast_ref::<PrepareError>().is_none() {
+                err.context(PrepareError::Unknown)
+            } else {
+                err
+            }
+        })?;
 
         std::fs::create_dir_all(self.target_dir())?;
         let res = f(&Build {

src/prepare.rs

@@ -390,6 +390,8 @@ pub enum PrepareError {
     /// Some of the dependencies do not exist anymore.
     #[error("the crate depends on missing dependencies: \n\n{0}")]
     MissingDependencies(String),
+    #[error("prepare failed without further details")]
+    Unknown,
 }
 
 #[cfg(test)]