fix: preserve original exception context in retry decorator

[armorbreak001]

Summary

Fixes #29

The retry decorator in samcli/lib/utils/retry.py now preserves the original exception context when all retries are exhausted.

Before: raise exc_raise(exc_raise_msg) — original exception was completely lost, making debugging impossible.

After: raise exc_raise(exc_raise_msg) from last_exc — Python exception chaining preserves the full traceback and original error message.

Changes

• Store each caught exception in last_exc
• Also break out of the retry loop for non-target exception types (avoid infinite loop on unexpected exceptions)
• Use raise ... from last_exc for proper exception chaining

Test

@retry(ValueError, attempts=2)
def flaky():
    raise ValueError("original error")
# Now shows: Exception: retry failed
# The above exception was the direct cause of: ValueError: original error