Add error injection either for this connection or for whole server.

Currently nothing for injecting errors into *another* connection... but that's
perhaps getting tricky-dicky for this point in time. Perhaps needed for events
if we don't do anything fancy.

Index: ndb-work/storage/ndb/src/mgmsrv/MgmtSrvr.cpp
===================================================================


storage/ndb/src/mgmsrv/MgmtSrvr.cpp:
  WL#3704 mgmapi timeouts: Add ndb_mgmd error injection
storage/ndb/src/mgmsrv/MgmtSrvr.hpp:
  WL#3704 mgmapi timeouts: Add ndb_mgmd error injection
storage/ndb/src/mgmsrv/Services.cpp:
  WL#3704 mgmapi timeouts: Add ndb_mgmd error injection
storage/ndb/src/mgmsrv/Services.hpp:
  WL#3704 mgmapi timeouts: Add ndb_mgmd error injection
storage/ndb/test/ndbapi/testMgm.cpp:
  WL#3704 mgmapi timeouts: Add ndb_mgmd error injection