Report write errors in glean-encode-scip2 instead of swallowing them

Summary:
`GleanJSONOutput::write` takes its writer by value and never flushes it, so the
`BufWriter` created in `build_json` is drained only by `Drop`, which discards
the error. A failing final write is therefore invisible and the tool exits 0
after logging `Wrote 1 file (N GiB)`. `std::fs::metadata(...).unwrap_or(0)` hid
a failing `stat` the same way.

Add an explicit `w.flush()?`, and propagate the `metadata` error.

Reviewed By: echistyakov

Differential Revision: D115445577

fbshipit-source-id: 2aefac56591c9c6e5ff3084b9123d22611c0c85b

Author: Wilfred

glean/lang/scip/indexer/scip_to_glean/cli/src/main.rs

@@ -261,7 +261,7 @@ fn build_json(args: BuildJsonArgs) -> Result<()> {
         let writer = std::io::BufWriter::new(write);
 
         shard.write(writer)?;
-        total_bytes += std::fs::metadata(&file).map(|m| m.len()).unwrap_or(0);
+        total_bytes += std::fs::metadata(&file)?.len();
     }
     info!(
         "Wrote {} {} ({})",

glean/lang/scip/indexer/scip_to_glean/cli/src/output.rs

@@ -543,6 +543,10 @@ impl GleanJSONOutput {
         )?;
         w.write_all(b"]\n")?;
 
+        // A buffered writer would otherwise flush on drop, which discards the
+        // error, so a failed final write would look like success.
+        w.flush()?;
+
         Ok(())
     }
 }