Segfault occurs if output text files can't be opened

[jakelangham]

Since there is no error handling in OutputSolutionData_txt, the program can segfault if it's unable to open a file.

It seems better to exit gracefully with an appropriate error message, so we should add this.

I don't believe the same thing will happen when outputting NetCDF since there's plenty of error handling in those routines.