Syntax errors in doctests

[mortenpi]

Documenter can't check for syntax errors in doctests, e.g.:

```jldoctest
julia> map(tuple, 1/(i+j) for i=1:2, j=1:2, [1:4;])
ERROR: syntax: invalid iteration specification
```

I am guessing this is because we parse before we eval, and we don't check for parsing errors.