Handle network errors gracefully during downloads

[Rani367]

Summary

Closes #409.

When internet connectivity is lost during downloads, the installer crashes with a raw Python traceback. This patch catches network exceptions and shows actionable error messages with recovery guidance instead.

Changes in urlcache.py:

• Add NetworkError exception class
• Add retry logic to get_size(), which previously had no retries at all (network errors during the initial HEAD request would crash immediately)
• get_block() now raises NetworkError after retries are exhausted instead of re-raising the raw exception

Changes in main.py:

• Wrap pre-partition download calls (load_ipsw, load_package) with interactive retry prompts, since no disk changes have been made at that point and retrying is safe
• Wrap post-partition do_install() calls with a clean exit message that tells the user to re-run the installer for automatic repair
• Add NetworkError handler to action_rebuild_vendorfw
• Add NetworkError catch in the global exception handler as a safety net

Before (raw traceback, no guidance):

Traceback (most recent call last):
  File "main.py", line ...
    ...
ConnectionResetError: [Errno 54] Connection reset by peer

After:

Pre-partition failure (interactive retry):

Download failed: Failed to connect to cdn.example.com after multiple retries
Please check your internet connection.
>> Retry (y/N):

Post-partition failure (clean exit with repair guidance):

Installation failed due to a network error.
  Download failed: lost connection to cdn.example.com

Please check your internet connection, then re-run the installer.
The installer will detect the incomplete installation and offer
to repair it.

Test plan

• NetworkError class imports correctly and is a proper Exception subclass
• get_size() retries 5 times with backoff on connection failure, then raises NetworkError
• get_block() retries 10 times (unchanged), then raises NetworkError instead of raw exception
• Exception chaining (from e) preserves original traceback in logs
• Both files pass py_compile syntax check
• Full install with stable network works unaffected (happy path)
• Kill network before partition creation, get retry prompt, restore network, retry succeeds
• Kill network after partition creation, get clean exit, re-run installer, repair offered