Add a space to separate sentences in the error message

[arbox]

@knu,

please consider reviewing this PR.

@marcandre added an informative error message, this PR adds a separator between messages to take them apart on the command line.

[olleolleolle]

This change improves the readability of the error message.

Thanks, @arbox!

Update: #14 is a reported issue about the build failure.
Update 2: It got fixed in 1 hour!

[marcandre]

Thanks!