MIEngine may issue multiple '-exec-continue' commands without waiting for break

Repro steps: Launch multiple processes under the MIEngine at the same time

Result: MIEngine may issue multiple '-exec-continue' commands, and then it doesn't ignore errors from the underlying debugger.

2: (12464) <--exec-continue
2: (12583) ->1015^done,stack=[frame={level="0",func="WebApplication1.Program.Main(string[] args)",clr-addr={module-id="{5e8d3911-1464-42c2-95f5-72e7b7f4a63c}",method-token="0x06000001",il-offset="0",native-offset="55"},addr="0x00007fc19f93e7d7",file="Program.cs",fullname="C:\\Users\\dazhao\\Documents\\Visual Studio 15\\Projects\\WebApplication3\\src\\WebApplication1\\Program.cs",line="14",col="9",end-line="14",end-col="10"},frame={level="1",func="[Native to Managed Transition]"}]
2: (12583) 1015: elapsed time 124
2: (12590) ->(gdb)
2: (12624) <--exec-continue
2: (12643) ->^running
2: (12643) ->(gdb)
2: (12643) ->^error,msg="Unknown Error: 0x9233000b"
2: (12643) ->(gdb)