Use more specific ipmi error codes

Most errors in the handler were returning an unsupported code and not a
more specific code articulating the failure.  Update the code to use
more specific errors.

Change-Id: I7f517d094245e8e1a6169f43582522889bb895d0
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/eth.cpp b/eth.cpp
index 2689fab..6dbf077 100644
--- a/eth.cpp
+++ b/eth.cpp
@@ -73,20 +73,20 @@
     {
         std::fprintf(stderr, "Invalid command length: %u\n",
                      static_cast<uint32_t>(*dataLen));
-        return IPMI_CC_INVALID;
+        return IPMI_CC_REQ_DATA_LEN_INVALID;
     }
 
     std::string device = NCSI_IF_NAME_STR;
     if (device.length() == 0)
     {
         std::fprintf(stderr, "Invalid eth string\n");
-        return IPMI_CC_INVALID;
+        return IPMI_CC_REQ_DATA_LEN_INVALID;
     }
 
     if ((sizeof(struct EthDeviceReply) + device.length()) > MAX_IPMI_BUFFER)
     {
         std::fprintf(stderr, "Response would overflow response buffer\n");
-        return IPMI_CC_INVALID;
+        return IPMI_CC_REQUESTED_TOO_MANY_BYTES;
     }
 
     // Fill in the response buffer.