transport: specialize throw message per transport backend

Specialize the error message per transport backend so consumers can
quickly identify which transport layer failed to initialize.

Without this change, a generic "Cannot initialize transport layer" error
is thrown regardless of which transport backend pldmd was built with.
Consumers have no way to distinguish whether the issue is a missing
mctp-demux service, a missing af-mctp service, or pldmd being built with
the wrong transport support.

Change-Id: I003785f05c7c492c040fa4df4a46bdf59a087081
Signed-off-by: Vishnunithyasoundhar S <vishnunithyasoundhar.s@intel.com>

common/transport.cpp

diff --git a/common/transport.cpp b/common/transport.cpp
index 4c19ced..6b7836a 100644
--- a/common/transport.cpp
+++ b/common/transport.cpp
@@ -125,7 +125,14 @@
     transport = transport_impl_init(impl, pfd, listening);
     if (!transport)
     {
-        throw std::runtime_error("Cannot initialize transport layer");
+#if defined(PLDM_TRANSPORT_WITH_MCTP_DEMUX)
+        throw std::runtime_error(
+            "Cannot initialize mctp-demux transport layer");
+#elif defined(PLDM_TRANSPORT_WITH_AF_MCTP)
+        throw std::runtime_error("Cannot initialize af-mctp transport layer");
+#else
+        throw std::runtime_error("Cannot initialize unknown transport layer");
+#endif
     }
 }