alias only parses the right hand side for tab completion help.
The assignment should happen whether or not the parse is successful because the
context may change by the time the alias is actually evaluated.
In particular, the 'set' command uses the loaded project for tab completion in 0.12.1.
When a .sbtrc file is processed, the project has not been loaded yet, so aliases
involving set fail. Wrapping the rhs in failOnException addresses this.