feat: Improve upload error message to show cause (#2765)

## Summary
- Improves error handling for invalid `--vcs-provider` parameter values
- When backend returns “400 Unsupported provider" error, shows
user-friendly message instead of generic API error

Before this PR:
```
WARN    2025-09-18 07:49:04.315821 +02:00 EXPERIMENTAL: The build subcommand is experimental. The command is subject to breaking changes and may be removed without notice in any release.
> Preparing for upload completed in 0.114s
  WARN    2025-09-18 07:49:05.135782 +02:00 Failed to upload 1 file:
  WARN    2025-09-18 07:49:05.135865 +02:00   - /Users/nelsonosacky/workspace/hackernews/android/app/build/outputs/apk/debug/app-debug.apk (API request failed)
error: Failed to upload any files
```

After this PR:
```
  WARN    2025-09-18 09:40:30.389061 +02:00 EXPERIMENTAL: The build subcommand is experimental. The command is subject to breaking changes and may be removed without notice in any release.
> Preparing for upload completed in 1.233s
  WARN    2025-09-18 09:40:32.341187 +02:00 Failed to upload 1 file:
  WARN    2025-09-18 09:40:32.342033 +02:00   - /Users/nelsonosacky/workspace/hackernews/android/app/build/outputs/apk/debug/app-debug.apk
  WARN    2025-09-18 09:40:32.342108 +02:00     Error: API request failed
  WARN    2025-09-18 09:40:32.342144 +02:00     Cause: sentry reported an error: Unsupported provider (http status: 400)
error: Failed to upload any files
```



Closes EME-301

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Daniel Szoke <7881302+szokeasaurusrex@users.noreply.github.com>

Author: 3 people

src/commands/build/upload.rs

@@ -325,7 +325,8 @@ pub fn execute(matches: &ArgMatches) -> Result<()> {
             }
         );
         for (path, reason) in errored_paths_and_reasons {
-            warn!("  - {} ({})", path.display(), reason);
+            warn!("  - {}", path.display());
+            warn!("    Error: {reason:#}");
         }
     }