Use anyhow more idiomatically (#31052)

https://github.com/zed-industries/zed/issues/30972 brought up another
case where our context is not enough to track the actual source of the
issue: we get a general top-level error without inner error.

The reason for this was `.ok_or_else(|| anyhow!("failed to read HEAD
SHA"))?; ` on the top level.

The PR finally reworks the way we use anyhow to reduce such issues (or
at least make it simpler to bubble them up later in a fix).
On top of that, uses a few more anyhow methods for better readability.

* `.ok_or_else(|| anyhow!("..."))`, `map_err` and other similar error
conversion/option reporting cases are replaced with `context` and
`with_context` calls
* in addition to that, various `anyhow!("failed to do ...")` are
stripped with `.context("Doing ...")` messages instead to remove the
parasitic `failed to` text
* `anyhow::ensure!` is used instead of `if ... { return Err(...); }`
calls
* `anyhow::bail!` is used instead of `return Err(anyhow!(...));`

Release Notes:

- N/A

crates/collab/src/auth.rs

@@ -3,7 +3,7 @@ use crate::{
     db::{self, AccessTokenId, Database, UserId},
     rpc::Principal,
 };
-use anyhow::{Context as _, anyhow};
+use anyhow::Context as _;
 use axum::{
     http::{self, Request, StatusCode},
     middleware::Next,
@@ -85,14 +85,14 @@ pub async fn validate_header<B>(mut req: Request<B>, next: Next<B>) -> impl Into
                 .db
                 .get_user_by_id(user_id)
                 .await?
-                .ok_or_else(|| anyhow!("user {} not found", user_id))?;
+                .with_context(|| format!("user {user_id} not found"))?;
 
             if let Some(impersonator_id) = validate_result.impersonator_id {
                 let admin = state
                     .db
                     .get_user_by_id(impersonator_id)
                     .await?
-                    .ok_or_else(|| anyhow!("user {} not found", impersonator_id))?;
+                    .with_context(|| format!("user {impersonator_id} not found"))?;
                 req.extensions_mut()
                     .insert(Principal::Impersonated { user, admin });
             } else {
@@ -192,7 +192,7 @@ pub async fn verify_access_token(
     let db_token = db.get_access_token(token.id).await?;
     let token_user_id = db_token.impersonated_user_id.unwrap_or(db_token.user_id);
     if token_user_id != user_id {
-        return Err(anyhow!("no such access token"))?;
+        return Err(anyhow::anyhow!("no such access token"))?;
     }
     let t0 = Instant::now();