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/main.cpp b/main.cpp
index 46e9feb..eaa82ab 100644
--- a/main.cpp
+++ b/main.cpp
@@ -49,7 +49,7 @@
     {
         std::fprintf(stderr, "*dataLen too small: %u\n",
                      static_cast<uint32_t>(*dataLen));
-        return IPMI_CC_INVALID;
+        return IPMI_CC_REQ_DATA_LEN_INVALID;
     }
 
     switch (reqBuf[0])