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/theme/src/default_colors.rs

@@ -288,7 +288,7 @@ impl TryFrom<StaticColorScaleSet> for ColorScaleSet {
     type Error = anyhow::Error;
 
     fn try_from(value: StaticColorScaleSet) -> Result<Self, Self::Error> {
-        fn to_color_scale(scale: StaticColorScale) -> Result<ColorScale, anyhow::Error> {
+        fn to_color_scale(scale: StaticColorScale) -> anyhow::Result<ColorScale> {
             scale
                 .into_iter()
                 .map(|color| Rgba::try_from(color).map(Hsla::from))