fix(custom,disk): catch fmt::format_error

[kylin-liwn]

What does this PR do?
Both modules call fmt::format() with user-controlled format strings. If a placeholder doesn't match any argument, fmt throws format_error. The custom module's catch was re-throwing instead of handling. Disk had none at all.

Either way the exception reaches GTK's signal handler — log fills with [error] traces and the bar can crash.

Handle it locally: log a warning, skip the affected path, fall back to plain text for the custom module.

Related issues
Closes #5275

Checklist

• Code is formatted with clang-format
• Builds locally (ninja -C build)
• Man page updated for any new/changed user-facing option (man/)
• Tested against the affected module(s)

[greptile-apps]

Confidence Score: 3/5

The PR is not safe to merge until the disk module's default positional format and the custom fallback's visibility handling are restored.

Findings

1.  Default disk format breaks ▶
2.  Fallback remains hidden ▶
3.  Failed paths count as rendered ▶

Summary

This PR catches user-triggered fmt::format_error exceptions in the custom and disk modules to prevent them from escaping GTK callbacks.

• Adds a plain-text fallback for custom-module label and tooltip formatting.
• Catches label and tooltip formatting failures per disk path.
• Accidentally removes the disk module's positional percentage argument, breaking its default format.
• Leaves failed disk paths and custom fallback output with incomplete rendering-state handling.

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