Better error message for Vec mono-connected to Seq

As discussed here: https://groups.google.com/forum/#!topic/chisel-users/dR5KO-9yZec

https://github.com/freechipsproject/chisel3/pull/739 only fixed this for bulk connect

**Type of issue**: bug report

**Impact**: no functional change

**Development Phase**: request

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?**

**What is the expected behavior?**

**Please tell us about your environment:**

**What is the use case for changing the behavior?**