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/migrations.rs

@@ -30,12 +30,11 @@ pub async fn run_database_migrations(
     for migration in migrations {
         match applied_migrations.get(&migration.version) {
             Some(applied_migration) => {
-                if migration.checksum != applied_migration.checksum {
-                    Err(anyhow!(
-                        "checksum mismatch for applied migration {}",
-                        migration.description
-                    ))?;
-                }
+                anyhow::ensure!(
+                    migration.checksum == applied_migration.checksum,
+                    "checksum mismatch for applied migration {}",
+                    migration.description
+                );
             }
             None => {
                 let elapsed = connection.apply(&migration).await?;