Use try_build instead of build when building terminal logger.

[dpc]

https://github.com/sile/sloggers/blob/master/src/terminal.rs#L123

term crate can fail to return a terminal (eg. if TERM is not set) and TerminalDecorator will just not do any logging which is confusing to the users.

Instead, use https://docs.rs/slog-term/2.2.0/slog_term/struct.TermDecoratorBuilder.html#method.try_build and in case of None use https://docs.rs/slog-term/2.2.0/slog_term/struct.PlainDecorator.html with io::stdout/io::stderr` or something like that that gives at least plain output.

Related: Stebalien/term#79

[dpc]

Actually I think I'm going to fix it in slog-term itself.

[sile]

Thank you for your advice. I fixed this problem in sloggers-v0.2.2.