Add error message in eval block (see #186)

[tlienart]

#186

• shows the error so that:

```julia:script/test
sqrt(-1)
```
\output{script/test}

gives

<pre><code class="language-julia">sqrt(-1)</code></pre> <pre><code>There was an error running the code: DomainError(-1.0, "sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).").</code></pre>

which can help for debugging.

[codecov]

Codecov Report

Merging #187 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #187   +/-   ##
=======================================
  Coverage   87.07%   87.07%           
=======================================
  Files          26       26           
  Lines        1037     1037           
=======================================
  Hits          903      903           
  Misses        134      134

Impacted Files	Coverage Δ
src/converter/md_blocks.jl	96.07% <100%> (ø)	⬆️

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