Unhelpful error for overlapping chains

**Description**
Embedding overlapping chains with the FixedEmbeddingComposite gives a DisconnectedChainError error, although the chains make sense individually.

**To Reproduce**
```
>>> qpu_2000q = DWaveSampler(solver={'topology__type': 'chimera'})
>>> FixedEmbeddingComposite(qpu_2000q, embedding={'a': [0, 4, 5], 'b': [0, 4, 1]}).sample_ising({}, {'ab': -1})
```
gives an error:
```
DisconnectedChainError: chain for a is not connected
```

From the user's perspective, ``'a': [0, 4, 5]`` is a good chain:
```
>>> qpu_2000q.adjacency[0]
{4, 5, 6, 7, 128}
```

**Expected behavior**
Would be helpful to get an error showing something like, OverlappingChainError with the overlapping nodes:
```
>>> set([0, 4, 5]) & set([0, 4, 1])
{0, 4}
```

**Environment:**
- OS: WIN
- Python version: 3.7.0

**Additional context**
I assume the chain is disconnected in the embedding because of prior use by a previous variable, but from the user's perspective each of the chains by itself is connected.