Better handling of error codes and error messages - show more information

[sypets]

Ideally, show different error messages for the libcurl errors or additionally show the exception message.

background

We used to assume, that errors with the same libcurl code are the same errors. But, for example for curl(60), there may be a wide range of different reasons, e.g.

exceptionMsg: with curl(60):

• "Peer's Certificate has expired."
• "Peer's Certificate issuer is not recognized."
• "Certificate type not approved for application." (example http://northseagrid.info/)
• "Peer's certificate issuer has been marked as not trusted by the user."
• "The certificate issuer's certificate has expired. Check your system date and time."
• "Certificate key usage inadequate for attempted operation."
• "Issuer certificate is invalid."
• "Peer's Certificate issuer is not recognized." (can be considered an edge case "false positive", chain incomplete, intermediate certs not sent by server, page can usually be loaded in browser without warning, if this is the only issue)

see #13 (comment)

We must also be aware, that under the hood something besides curl might be used, so we should be able to handle a wide range of error codes, exceptions etc.

Example:

,"errorType":"libcurlErrno","errno":60,"exceptionMsg":"Peer's Certificate has expired."

The displayed error message is:

SSL / TLS certificate not OK.