Better error messages

[richfitz]

With this set up:

packages: rmarkdown
targets:
  manuscript.docx:
    command: render("manuscript.Rmd", "word_document", "manuscript.docx")
    depends: figures

The error message generated is:

Error in .remake_add_targets_implied(obj) : 
  Implicitly created targets must all be files:
 - word_document: (in manuscript.docx)

because word_document should be in I(). Perhaps link message to a helpfile that describes I and implicit targets. Perhaps suggest I if no extension is present on the name.

[richfitz]

cc: @dfalster, @RemkoDuursma