Improve error handling when PathSource is relative

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.

This commit changes the error message to say what is actually wrong.

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)
 }