[dtls] map OT errors from send callback to MBEDTLS_ERR_NET_SEND_FAILED (#3259)

Credit to OSS-Fuzz.

src/core/meshcop/dtls.cpp

@@ -534,7 +534,8 @@ int Dtls::HandleMbedtlsTransmit(const unsigned char *aBuf, size_t aLength)
         break;
 
     default:
-        assert(false);
+        otLogWarnMeshCoP("Dtls::HandleMbedtlsTransmit: %s error", otThreadErrorToString(error));
+        rval = MBEDTLS_ERR_NET_SEND_FAILED;
         break;
     }
 

src/core/meshcop/dtls.hpp

@@ -41,6 +41,7 @@
 #include <mbedtls/entropy.h>
 #include <mbedtls/entropy_poll.h>
 #include <mbedtls/error.h>
+#include <mbedtls/net_sockets.h>
 #include <mbedtls/ssl.h>
 #include <mbedtls/ssl_cookie.h>