Implement `NetworkError`

[codebestia]

Description

Raised when a request fails due to a client-side network issue — DNS failure, connection refused, timeout — rather than an API error response. Wraps the underlying httpx exception for context.

Proposed Steps

• Define NetworkError(ShadeError) in errors.py.
• Add original_error: Exception attribute.
• Catch httpx.ConnectError, httpx.TimeoutException, httpx.ReadError in the HTTP client and re-raise as NetworkError.

Acceptance Criteria

• A connection timeout raises NetworkError with original_error set to the httpx exception.
• str(error) includes a human-readable description of the network failure.
• Does not suppress KeyboardInterrupt or SystemExit.

[yeiner-45]

Hi! I'm highly interested in this issue. Properly wrapping third-party library exceptions (like httpx) into domain-specific errors is crucial for building a resilient SDK. I have strong experience with Python's exception handling hierarchies, custom error definitions, and object inheritance. I can cleanly define the NetworkError class in errors.py, implement the specific try/except blocks in the HTTP client while avoiding catching system exits, and ensure human-readable error messages are preserved. I'm ready to tackle this right away!