Change error message with Shiny app
<p>Simply, I have a Shiny app, where users can select various combinations of values in a data set and produces a graph. However for some of those combinations there is no data and Shiny produces an error message:</p>

<p><code>Error: 'from' must be length 1</code></p>

<p>How can I replace this error message with a more informative message, like:</p>

<p><code>Sorry, there is no data for you requested combination. Please change your input selections</code></p>

<p>Thanks.</p>