crypto/x509: improve error message when signature algorithm is unsupported

[tobiasworkstech]

Fixes #7735

When certificate chain verification fails because the CA's signature
algorithm is not supported, the error was wrapped in an
UnknownAuthorityError producing a misleading message:

x509: certificate signed by unknown authority (possibly because of
"x509: cannot verify signature: algorithm unimplemented" while trying
to verify candidate authority certificate "CA Name")

The phrase "signed by unknown authority" is confusing: the authority
certificate is known and found, but its signature algorithm cannot
be verified by this build of Go. The inner error string is accurate
but buried and obscured by the outer phrasing.

This change detects when hintErr is ErrUnsupportedAlgorithm and
produces a clearer, direct message instead:

x509: certificate signed by unknown authority (the signature algorithm
of authority certificate "CA Name" is not supported)

The existing expectHashError test is updated to match.

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Patch Set 1:

(1 comment)

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