Make file download fail-safe and retry on transient errors

[jimexist]

What

Fixes a crash when a translation file download fails, and adds bounded retry for transient download errors.

Why

A failed download (e.g. an expired or forbidden S3 link) crashed the CLI with an unhelpful stack trace instead of a proper error message:

java.lang.IllegalArgumentException: json string can not be null or empty
	at com.jayway.jsonpath.internal.Utils.notEmpty(Utils.java:401)
	at com.jayway.jsonpath.internal.ParseContextImpl.parse(ParseContextImpl.java:36)
	at io.simplelocalize.cli.client.SimpleLocalizeClient.throwOnError(SimpleLocalizeClient.java:201)
	at io.simplelocalize.cli.client.SimpleLocalizeClient.downloadFile(SimpleLocalizeClient.java:133)
	...

Root cause: for file downloads the response body is a Path (from BodyHandlers.ofFile), which safeCastHttpBodyToString turned into "", and JsonPath.parse("") throws before SUPPRESS_EXCEPTIONS can apply. The same crash affected any endpoint returning an error with an empty body.

Changes

• throwOnError never crashes on unparseable bodies: error-message extraction skips blank bodies and catches any JsonPath failure, falling back to Unknown error, HTTP Status: <code>.
• Path bodies are read for error messages: failed downloads now surface the server's actual error message (e.g. link expired) instead of a generic one.
• No error payload left on disk: BodyHandlers.ofFile(..., TRUNCATE_EXISTING) writes the error response to the target translation file on failure; it is now deleted instead of being left behind masquerading as a translation file.
• Downloads retry on transient errors: up to 3 attempts with linear backoff (1s, 2s) on IOException, HTTP 429, and 5xx. Other 4xx (403/404) fail fast since retrying can't help.

Notes for reviewers

• Existing behavior for permanent errors is unchanged: an ApiRequestException with the extracted (or fallback) message.
• One known pre-existing limitation not addressed here: a failed download still truncates a pre-existing local file before the failure is detected. Downloading to a temp file and moving on success would fix that; happy to do it in a follow-up.

Tests

Added coverage in SimpleLocalizeClientTest (mock-server based):

• download fails with empty body → clean ApiRequestException, no leftover file (the reported crash)
• download fails with JSON error body → message extracted from $.msg
• server returns 500 twice then 200 → download succeeds on the third attempt with correct content
• server returns 503 three times → gives up after max attempts, no leftover file

Full suite passes: 88 tests, 0 failures.

🤖 Generated with Claude Code

[jpomykala]

Hi @jimexist , thanks for the PR. I will review and merge it later this week. 😊