Improve error handling when PathSource is relative (#14854)

When editing dependencies with cargo, if a relative PathSource is
supplied cargo panics with "both paths are absolute". This is the
opposite of what's actually wrong leading to confusion.

Instead, use the same error formatting we use in other diff_paths calls
and return that error instead of panicking.

Author: weihanglo

src/cargo/util/toml_mut/dependency.rs

@@ -748,7 +748,8 @@ fn path_field<'a>(
     } else {
         Cow::Borrowed(crate_root)
     };
-    let relpath = pathdiff::diff_paths(&source.path, relative_to).expect("both paths are absolute");
+    let relpath = pathdiff::diff_paths(&source.path, relative_to)
+        .expect("PathSource::path and workspace path must be absolute");
     let relpath = relpath.to_str().unwrap().replace('\\', "/");
     Ok(relpath)
 }