Fix core dump when invalid MAC is configured

When configuring an invalid MAC, a runtime error is thrown
with the existing implementation, which causes a core dump
and phosphor-networkd does not respond when further requests
are made.

This commit avoids the core dump that's caused.

Signed-off-by: Asmitha Karunanithi <asmitk01@in.ibm.com>
Change-Id: I5cb5a25e91a685e1bb2f4bb80d3d8f8c483f56a1
diff --git a/util.cpp b/util.cpp
index cb9e720..0c5dbff 100644
--- a/util.cpp
+++ b/util.cpp
@@ -631,7 +631,7 @@
     struct ether_addr* mac = ether_aton(str);
     if (mac == nullptr)
     {
-        throw std::runtime_error("Invalid mac address string");
+        throw std::invalid_argument("Invalid MAC Address");
     }
     return *mac;
 }