SelectDataset From with Error

[marshall-mcmullen]

Describe the bug
If you use SelectDataset From and pass it another SelectDataset that has an error already set on it, it seems to ignore the error on the subselect instead of propagating it.

To Reproduce

func TestSelectWithSubSelectError(t *testing.T) {

    ds := goqu.From("test").As("t").SetError(fmt.Errorf("An error"))
    assert.NotNil(t, ds.Error())

    ds2 := goqu.From(ds)                                                                                 
    assert.NotNil(t, ds2.Error())
}

If you run this test the first assertion passes, but the second assertion fails.

        	Error:      	Expected value not to be nil.
        	Test:       	TestSelectWithSubSelectError

Expected behavior
I would expect using a SelectDataset as a subselect on another SelectDataset would honor the error and propagate it rather than swallow the error. Obviously the caller can check this scenario with gs.Error() before using it in a subselect, but I think that puts too much burden on the caller IMO and this is something goqu should handle.

Dialect:

• postgres
• mysql
• sqlite3

Additional context
Add any other context about the problem here.

[marshall-mcmullen]

I've created a PR: #184 to address this issue in a way that made sense to me. @doug-martin please let me know what you think.

[marshall-mcmullen]

Thanks so much for getting this merged @doug-martin