Prevent error messages from overflowing to the left.

[rehno-lindeque]

When tiling a text-editor next to elm-reactor the error message often gets pushed off the left side of the screen which makes it difficult to read.

This little patch keeps the centered alignment while preventing overflow on the left-hand side.

Before:

After:

( http://stackoverflow.com/a/33455342 explains the margin: auto trick, in case that's helpful )

Thanks!

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.