Currently, bitcoind reports an error when it wants to warn the user (eg, when hitting an outdated version):

bitcoin-cli getinfo
{
    "version" : 110100,
    "protocolversion" : 70002,
    "walletversion" : 60000,
    "balance" : 1.69743172,
    "blocks" : 385894,
    "timeoffset" : 0,
    "connections" : 8,
    "proxy" : "",
    "difficulty" : 72722780642.54718018,
    "testnet" : false,
    "keypoololdest" : 1423141169,
    "keypoolsize" : 10001,
    "unlocked_until" : 0,
    "paytxfee" : 0.00000000,
    "relayfee" : 0.00005000,
    "errors" : "Warning: This version is obsolete; upgrade required!"
}

Now, this is confusing. A warning is not an error. Warnings should be reported, but should not stop processing as errors do.

The problem with the current behavior is that it causes libraries to bail: common behavior is to check for the presence of errors in the result to report an error to the calling code. An example is python-bitcoinrpc: the library raises an exception, which interrupts the normal processing of the result. This happily breaks production systems due to a non-criticial warning.

Cleanest way would be to add a warnings field in the results. However, this would require all libs to be updated so that the warning is not silently swallowed but exposed to the user.