Improve display of error message in sql export if type conversion fails

Describe the bug
In SQL export, you can choose the SQL type (varchar, numeric, date...) of a column. If the type of the data in one cell does not fit (ex : a word in a "numeric" column), Openrefine display a error message in a blank HTML page. It would be better to display this message in the OR interface (or to display a warning before trying to export the data).

To Reproduce
Steps to reproduce the behavior:

1. Create a dataset with a word in the 1st colum
2. try to export it in SQL
3. in export options, define the 1st column as "Numeric"

Current Results
What results occured or were shown.

Expected behavior
A clear and concise description of what you expected to happen or to show.

Screenshots
If applicable, add screenshots to help explain your problem.
http://127.0.0.1:3333/command/core/export-rows/clipboard.sql

Desktop (please complete the following information):

• OS: Mac OS
• Browser Version: Chrome
• JRE or JDK Version:1.8.0_152

OpenRefine (please complete the following information):

• Version : OpenRefine 3.0

Datasets
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Additional context
Add any other context about the problem here.

Start with the SQL export flow invoked through the export-rows/clipboard.sql endpoint and reproduce the failure using a word in a column configured as Numeric. Trace where the type-conversion error becomes a blank HTML page; done means the error or warning is shown in the OpenRefine interface instead.