SPV_FAILED_MATCH error mutes DiagnosticStream.

SPV_FAILED_MATCH error mutes DiagnosticStream.

The DiagnosticStream will not emit the accumulated message
text if the error is SPV_FAILED_MATCH.

Change various interfaces to accept the intended error
code instead of a boolean "is_optional".  This allows
us to avoid repeating the following type of logic deep
inside helper methods:

  if (is_optional) return SPV_FAILED_MATCH;
  return diagnostic() << " message text ";