bug: inconsistent missing resource handling

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### Description

There is a slight inconsistency in how `capa` handles missing default resources. When default rules are missing, the tool provides a clean error message and exits gracefully. However, when default signatures are missing, it identifies the issue but then raises an unhandled `OSError` resulting in a crash/traceback.

### Steps to Reproduce

1. Install `capa` as a library (`pip install .`).
2. Run `capa` against a sample providing only the rules path: `capa -r rules`

**Expected behavior:**

`capa` should log a clean error for missing signatures and exit gracefully, similar to the "default embedded rules not found" error. Something like what happens when you run `capa` without a rules folder:

```text
ERROR capa: default embedded rules not found! (maybe you installed capa as a library?) main.py:495
ERROR capa: provide your own rule set via the `-r` option. main.py:496
```

Exits with code `E_MISSING_RULES` (10), and no `OSError`.

**Actual behavior:**

The tool logs a signature error but crashes with an unhandled `OSError`.

```
ERROR capa: Using default signature path, but it doesn't exist. Please install the signatures first:
https://github.com/mandiant/capa/blob/master/doc/installation.md#method-2-using-capa-as-a-python-library.
Traceback (most recent call last):
File "/opt/homebrew/bin/capa", line 6, in
sys.exit(main())
.
.
.
File "/opt/homebrew/lib/python3.14/site-packages/capa/main.py", line 842, in get_signatures_from_cli
raise IOError(f"signatures path {args.signatures} does not exist or cannot be accessed")
OSError: signatures path /opt/homebrew/lib/python3.14/site-packages/sigs does not exist or cannot be accessed
```

Exits with the generic code 1

### Versions

- capa 9.4.0
- Standalone tool (installed via `pip install .`) with rules and sigs as git submodules
- Python 3.14
- macOS (Darwin)

Start in capa/main.py at get_signatures_from_cli and compare its missing-signatures handling with the missing-rules handling around the reported main.py lines 495-496. Reproduce the command using only the rules path, then verify that missing signatures produce a clean error, exit with E_MISSING_RULES (10), and do not raise an OSError traceback.

Written by the indexing model from the issue text.