pageserver: improved synthetic size & find_gc_cutoff error handling (#8051)

## Problem

This PR refactors some error handling to avoid log spam on
tenant/timeline shutdown.

- "ignoring failure to find gc cutoffs: timeline shutting down." logs
(https://github.com/neondatabase/neon/issues/8012)
- "synthetic_size_worker: failed to calculate synthetic size for tenant
...: Failed to refresh gc_info before gathering inputs: tenant shutting
down", for example here:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-8049/9502988669/index.html#suites/3fc871d9ee8127d8501d607e03205abb/1a074a66548bbcea

Closes: https://github.com/neondatabase/neon/issues/8012

## Summary of changes

- Refactor: Add a PageReconstructError variant to GcError: this is the
only kind of error that find_gc_cutoffs can emit.
- Functional change: only ignore shutdown PageReconstructError variant:
for other variants, treat it as a real error
- Refactor: add a structured CalculateSyntheticSizeError type and use it
instead of anyhow::Error in synthetic size calculations
- Functional change: while iterating through timelines gathering logical
sizes, only drop out if the whole tenant is cancelled: individual
timeline cancellations indicate deletion in progress and we can just
ignore those.

pageserver/src/http/routes.rs

@@ -1135,7 +1135,10 @@ async fn tenant_size_handler(
             &ctx,
         )
         .await
-        .map_err(ApiError::InternalServerError)?;
+        .map_err(|e| match e {
+            crate::tenant::size::CalculateSyntheticSizeError::Cancelled => ApiError::ShuttingDown,
+            other => ApiError::InternalServerError(anyhow::anyhow!(other)),
+        })?;
 
     let mut sizes = None;
     let accepts_html = headers
@@ -1143,9 +1146,7 @@ async fn tenant_size_handler(
         .map(|v| v == "text/html")
         .unwrap_or_default();
     if !inputs_only.unwrap_or(false) {
-        let storage_model = inputs
-            .calculate_model()
-            .map_err(ApiError::InternalServerError)?;
+        let storage_model = inputs.calculate_model();
         let size = storage_model.calculate();
 
         // If request header expects html, return html