Validate a base URL (baseUrlProblem) and throw AwValidationError when it is not
usable: a configuration error, never an AwNetworkError, which a caller may retry
as transient. The userinfo is redacted from the message. The engine runs it on the
raw configured value, before it strips trailing slashes, so a bad
--base-url ftp://x names the value the user passed rather than
ftp://x/api/v2/... from the transport.
Validate a base URL (baseUrlProblem) and throw AwValidationError when it is not usable: a configuration error, never an AwNetworkError, which a caller may retry as transient. The userinfo is redacted from the message. The engine runs it on the raw configured value, before it strips trailing slashes, so a bad
--base-url ftp://xnames the value the user passed rather thanftp://x/api/v2/...from the transport.