Catch more errors in backoff
This one is slipping through backoff:
E0111 14:37:31.497518 1817971 fetcher.go:292] https://ct2025-a.trustasia.com/log2025a: GetRawEntries() failed: failed to read response body: unexpected EOF [1724/1724]
E0111 14:37:31.516126 1817971 fetcher.go:292] https://ct2025-a.trustasia.com/log2025a: GetRawEntries() failed: failed to read response body: unexpected EOF
E0111 14:37:31.534967 1817971 fetcher.go:292] https://ct2025-a.trustasia.com/log2025a: GetRawEntries() failed: failed to read response body: unexpected EOF