workspace add: add filename context to FS error

At work, a user encountered a panic upon attempting to create a dir at
the line in the diff below, but it turned out to be difficult to debug
because I didn't know what the path was. There already is a mechanism to
add path context in the lib crate; make it available in the cli crate as
well, and use the mechanism to add path context to "workspace add".

cli/src/command_error.rs

@@ -206,6 +206,12 @@ impl From<io::Error> for CommandError {
     }
 }
 
+impl From<jj_lib::file_util::PathError> for CommandError {
+    fn from(err: jj_lib::file_util::PathError) -> Self {
+        user_error(err)
+    }
+}
+
 impl From<config::ConfigError> for CommandError {
     fn from(err: config::ConfigError) -> Self {
         config_error(err)

cli/src/commands/workspace.rs

@@ -21,6 +21,7 @@ use clap::Subcommand;
 use itertools::Itertools;
 use jj_lib::commit::CommitIteratorExt;
 use jj_lib::file_util;
+use jj_lib::file_util::IoResultExt;
 use jj_lib::object_id::ObjectId;
 use jj_lib::op_store::{OpStoreError, WorkspaceId};
 use jj_lib::operation::Operation;
@@ -137,7 +138,7 @@ fn cmd_workspace_add(
     if destination_path.exists() {
         return Err(user_error("Workspace already exists"));
     } else {
-        fs::create_dir(&destination_path).unwrap();
+        fs::create_dir(&destination_path).context(&destination_path)?;
     }
     let name = if let Some(name) = &args.name {
         name.to_string()

lib/src/file_util.rs

@@ -31,7 +31,7 @@ pub struct PathError {
     pub error: io::Error,
 }
 
-pub(crate) trait IoResultExt<T> {
+pub trait IoResultExt<T> {
     fn context(self, path: impl AsRef<Path>) -> Result<T, PathError>;
 }