refact(log): consolidate some duplicate log statements

[ankostis]

Use string interpolation for printing log arguments instead of another multiple log statements.

[ankostis]

Note that the log-statement about change in Application.config observer fires even if the configuration dictionary is empty.
And this always happen at least once, when the class gets constructed (configurations are loaded later, on initialize()) so debug-logs have some redundant info.

Question: Would it make sense to avoid a debug log-line if config is empty?
(it might still be useful when debugging cases where configs come up empty, while they shouldn't, [edit:] but there are other way to discover that)

[ankostis]

Also this log-statement in traitlets.config.Loader#718 deserves to be error-level. On a respective problem for flags, below, it raises.

Should i fix add this in this PR?