Better context information handling for exceptions.

Better context information handling for exceptions.

- GromacsException now has a prependContext() method that can be used in
  a catch block to add additional context information to the exception
  before rethrowing it.
- It is possible to nest exceptions instead of embedding all information
  into the exception message string.

Information added by these new methods is currently not easily
accessible outside the exception implementation, but it is not needed
either.  Such access can be implemented once it is more clear what are
the requirements.

Closes #985.

Change-Id: I2b9ed1d11ee3cb36cc7da927cf6730c6f8c353b2