str_trunc behaves unexpectedly when width is shorter than elipsis

[ClaytonJY]

For example

> sapply(3:1, function(x) str_trunc("blahblahblah", x))
[1] "..."             "blahblahblah..." "blahblahbla..." 
> sapply(5:1, function(x) str_trunc("blahblahblah", x))
[1] "bl..."           "b..."            "..."             "blahblahblah..." "blahblahbla..." 
> sapply(5:1, function(x) str_trunc("blahblahblah", x, side = "left"))
[1] "...ah"           "...h"            "...blahblahblah" "...blahblahblah" "...lahblahblah" 
> sapply(5:1, function(x) str_trunc("blahblahblah", x, side = "center"))
[1] "b...h"                       "b...blahblahblah"            "...blahblahblah"             "...blahblahblah"            
[5] "blahblahblah...blahblahblah"
> sapply(5:1, function(x) str_trunc("blahblahblah", x, ellipsis = ".."))
[1] "bla.."          "bl.."           "b.."            ".."             "blahblahblah.."
> sapply(5:1, function(x) str_trunc("blahblahblah", x, ellipsis = "...."))
[1] "b...."            "...."             "blahblahblah...." "blahblahbla...."  "blahblahbl...."  

I'm not sure what should happen when width < str_length(elipsis) (warning? error? NA?), but making the string longer can't be right.

Session Info

> devtools::session_info()
Session info ----------------------------------------------------------------------------------------------------------------------------
 setting  value                       
 version  R version 3.4.0 (2017-04-21)
 system   x86_64, linux-gnu           
 ui       RStudio (1.0.143)           
 language (EN)                        
 collate  en_US.UTF-8                 
 tz       America/New_York            
 date     2017-05-09                  

Packages --------------------------------------------------------------------------------------------------------------------------------
 package  * version date       source        
 devtools   1.12.0  2016-12-05 CRAN (R 3.4.0)
 digest     0.6.12  2017-01-27 CRAN (R 3.4.0)
 magrittr * 1.5     2014-11-22 CRAN (R 3.4.0)
 memoise    1.1.0   2017-04-21 CRAN (R 3.4.0)
 stringi    1.1.5   2017-04-07 CRAN (R 3.4.0)
 stringr  * 1.2.0   2017-02-18 CRAN (R 3.4.0)
 withr      1.0.2   2016-06-20 CRAN (R 3.4.0)

(not dev, but this function hasn't changed in awhile)

[ClaytonJY]

I'd be happy to fix this in a PR (maybe add on to #165), but I'm unsure what the right behavior is; truncate the elipsis? e.g.

> str_trunc("foobar", 2)
[1] ".."

[hadley]

I think an error would be most appropriate. A PR would be great!