[C++] Fix exception message in createProxyBuilder().
[C++] Fix exception message in createProxyBuilder().

Change-Id: Ia7e11519db0a266d8a32acec52809bd40f7499bd

clive-jevons/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr,bmwcarit/joynr,clive-jevons/joynr