Errors

What the number means.

Most download failures report an HTTP code. Knowing which side the fault is on saves the time you would spend retrying.

HTTP codes

400
Bad request — the server did not understand the URL
403
Forbidden — the file exists but you are not allowed to fetch it
404
Not found — the link is dead or was mistyped
414
URL too long for the server to accept
500
Server error — the fault is on the far end, try later
501
Not implemented — the server refuses the method
502
Bad gateway — a proxy in between failed
503
Service unavailable — the server is overloaded or down

Which are yours to fix

Codes in the 400s are about the request: a dead link, a file behind a login, an address that was cut when copied. Retrying will not help; the link needs to change.

Codes in the 500s are about the server. Retrying later often does help, and the scheduler exists partly for that.

Each code has its own page in the documentation, with what the program does automatically.