Use correct classed error for "same name, different user" case

[juliasilge]

Closes #807

When a user was trying to write a new pin that had the same name as an existing pin belonging to a different user, we weren't correctly adding class = "pins_pin_missing" to get the right behavior in the tryCatch():

pins-r/R/board_connect.R

Lines 276 to 278 in ed3ac3b

	pins_pin_missing = function(e) {
	rsc_content_create(board, name, metadata, access_type = access_type)$guid
	}

This PR adds the correct classed condition.

[juliasilge]

@stevenolen Would you be up for installing this PR and trying again the code that you had trouble with? You can install with devtools::install_github("rstudio/pins-r@fix-error-class").

This PR should fix the situation where you, for example, write your own pin named "steve.nolen/test". You will still see an error if you try to write a pin named "test"; I think we want to keep that an error for now.

[github-actions]

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