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/env.rs

@@ -1,4 +1,4 @@
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result};
 use std::fs;
 use std::path::Path;
 
@@ -6,8 +6,8 @@ pub fn get_dotenv_vars(current_dir: impl AsRef<Path>) -> Result<Vec<(String, Str
     let current_dir = current_dir.as_ref();
 
     let mut vars = Vec::new();
-    let env_content = fs::read_to_string(current_dir.join(".env.toml"))
-        .map_err(|_| anyhow!("no .env.toml file found"))?;
+    let env_content =
+        fs::read_to_string(current_dir.join(".env.toml")).context("no .env.toml file found")?;
 
     add_vars(env_content, &mut vars)?;