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/fs/src/fs_watcher.rs

@@ -23,7 +23,7 @@ impl FsWatcher {
 }
 
 impl Watcher for FsWatcher {
-    fn add(&self, path: &std::path::Path) -> gpui::Result<()> {
+    fn add(&self, path: &std::path::Path) -> anyhow::Result<()> {
         let root_path = SanitizedPath::from(path);
 
         let tx = self.tx.clone();
@@ -78,7 +78,7 @@ impl Watcher for FsWatcher {
         Ok(())
     }
 
-    fn remove(&self, path: &std::path::Path) -> gpui::Result<()> {
+    fn remove(&self, path: &std::path::Path) -> anyhow::Result<()> {
         use notify::Watcher;
         Ok(global(|w| w.watcher.lock().unwatch(path))??)
     }
@@ -130,6 +130,6 @@ pub fn global<T>(f: impl FnOnce(&GlobalWatcher) -> T) -> anyhow::Result<T> {
     });
     match result {
         Ok(g) => Ok(f(g)),
-        Err(e) => Err(anyhow::anyhow!("{}", e)),
+        Err(e) => Err(anyhow::anyhow!("{e}")),
     }
 }