Surface API error messages and standardize HTTP status fallback messages

[CarterSmarty]

Summary

Implements the standard error message scheme shared across all Smarty SDKs: every HTTP error status now surfaces the message from the API's JSON error body ({"errors":[{"message":"..."}]}) when one is present, and falls back to the standard per-status message list otherwise (304, 400, 401, 402, 403, 408, 413, 422, 429, 500, 502, 503, 504, plus an "unexpected status code" default).

In this SDK

• Added 403/408/502/504 and unknown-status handling (these previously produced no error at all); new RequestTimeoutError/BadGatewayError classes.
• Malformed JSON error bodies no longer raise JSON::ParserError; they fall back to the standard message.
• Constants standardized (old ones contained literal embedded newlines mid-message).

Validation

• Unit tests cover every handled status: API message preferred, exact standard fallback on empty/malformed/unusable bodies, unknown status codes, and unchanged 304/ETag behavior.
• Verified against the live API with real credentials: 400 (missing street), 401 (bad credentials), 404 (unexpected-status path), and 422 (out-of-range latitude) each surfaced the server's own message through this SDK's full client chain.

Related PRs (same change in every SDK)

• smartystreets-java-sdk
• smartystreets-go-sdk
• smarty-rust-sdk
• smartystreets-python-sdk
• smartystreets-ruby-sdk
• smartystreets-php-sdk
• smartystreets-javascript-sdk
• smartystreets-dotnet-sdk
• smartystreets-ios-sdk