Turn warning for absence of main reactor into an informational message

[edwardalee]

Currently, if you have a .lf file with no main reactor, you get a warning:

WARNING: The given Lingua Franca program does not define a main reactor. Therefore, no code was generated.

This gets flagged as a yellow caution symbol in the IDE. I don't think this should be a warning. It should be reported as "Information", but our ErrorReporter interface does not have support for this. It should be straightforward to extend it, so I'm marking this as a good first issue.

[cmnrd]

I am wondering what the error reporter should do with the info message, though. Print it to the command line? I think there is no obvious way of showing info messages in an IDE. However, if info messages are always printed to the command line, we could just print them directly, right?

[edwardalee]

Eclipse has an "info" icon that it can put where it puts the warning or error icon in the left margin.

[lhstrh]

Fixed by @zekailin00 in #1192.