add c++ throw where jdksavdecc_descriptor_XXX_read() is called and catch in end_station_imp.cpp where background read is performed.
add avdecc_error.h and throw avdecc_read_descriptor_error when PDU read fails
Merge remote-tracking branch 'asi/master' into staging
add development sequence diagram
Merge pull request #107 from andrew-elder/staging

add throw/catch to descriptor read operations