Mid-handshake errors that occur before certificate verification
currently look like this:
```
TLS handshake failed: cert verification failed - Invalid certificate verification context [WRONG_VERSION_NUMBER]
```
Despite no certificate even being received yet, the error complains
about a failed verification. The cause here is that `cert verification
failed` is only omitted if the verification result is `OK`. The default
in BoringSSL before verification runs is `INVALID_CALL`, however.
`INVALID_CALL` is set/returned in these places:
- 44b3df6f03/src/ssl/internal.h (L3904)
- 44b3df6f03/src/ssl/ssl_session.cc (L396)
- 44b3df6f03/src/ssl/ssl_x509.cc (L713)
It is not used anywhere else as a verification result code. To improve
the error message, this commit adds `INVALID_CALL` as a verification
result for which no additional error is dislayed.
Kornel