Add context for workspace env var resolution errors (meta-pytorch#879)

Summary:
Pull Request resolved: meta-pytorch#879

Previously, if the env var we read from to locate the workspace isn't
set, we'd just get a cryptic `NotPresent` error back from the client,
with not clue as to where it came from.

Add context to make it very clear.

Reviewed By: highker

Differential Revision: D80267336

fbshipit-source-id: 844fe3d2c7be622ddcb3eef3a072ebf6635dbd30

Author: andrewjcg

monarch_hyperactor/src/code_sync/rsync.rs

@@ -358,7 +358,9 @@ impl Handler<RsyncMessage> for RsyncActor {
         }: RsyncMessage,
     ) -> Result<(), anyhow::Error> {
         let res = async {
-            let workspace = workspace.resolve()?;
+            let workspace = workspace
+                .resolve()
+                .context("resolving workspace location")?;
             let (connect_msg, completer) = Connect::allocate(cx.self_id().clone(), cx);
             connect.send(cx, connect_msg)?;
             let (listener, mut stream) = try_join!(

monarch_hyperactor/src/code_sync/workspace.rs

@@ -8,6 +8,7 @@
 
 use std::path::PathBuf;
 
+use anyhow::Context;
 use anyhow::Result;
 use serde::Deserialize;
 use serde::Serialize;
@@ -22,7 +23,9 @@ impl WorkspaceLocation {
     pub fn resolve(&self) -> Result<PathBuf> {
         Ok(match self {
             WorkspaceLocation::Constant(p) => p.clone(),
-            WorkspaceLocation::FromEnvVar(v) => PathBuf::from(std::env::var(v)?),
+            WorkspaceLocation::FromEnvVar(v) => PathBuf::from(
+                std::env::var_os(v).with_context(|| format!("workspace env var not set: {}", v))?,
+            ),
         })
     }
 }