ENH: Warn when provided dataset to wtf does not exist

[adswa]

it now says:

 datalad wtf -d ../nada                                                                       1 ↵
[WARNING] No dataset found at /tmp/nada. Reporting on the dataset is not attempted. [wtf(/tmp/nada)] 
# WTF
# WTF
## configuration <SENSITIVE, report disabled by configuration>
## datalad 
  - full_version: 0.12.3.dev547-g6a60c
  - version: 0.12.3.dev547
## dependencies 

fixes #4100

[codecov]

Codecov Report

Merging #4331 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4331   +/-   ##
=======================================
  Coverage   88.85%   88.85%           
=======================================
  Files         285      285           
  Lines       37491    37492    +1     
=======================================
+ Hits        33313    33314    +1     
  Misses       4178     4178           

Impacted Files	Coverage Δ
datalad/plugin/wtf.py	82.27% <100.00%> (+0.07%)	⬆️

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[mih]

I think this is good. Thx!

[yarikoptic]

Wouldn't this be worthwhile for maint?

[adswa]

Can we revert merging this? I just spotted in #4338 that it broke a test

[kyleam]

Can we revert merging this? I just spotted in #4338 that it broke a test

Oh my, I'm sorry. I thought I checked for failures, but I guess I overlooked this. This is my fault, so let me look into fixing it on top, but if for some reason that's not quick/easy, then I'd say we should revert.

[adswa]

nah, my fault (but I'm also fine blaming @mih) -- I bet I force-pushed a final change to hastily. Stupidly, that test is skipped when I run it locally.

[kyleam]

Hopefully resolved with gh-4339.

[kyleam]

Wouldn't this be worthwhile for maint?

I'm okay with this change only being in master.

[adswa]

thanks @kyleam!