tfdiags: Unwrap NonFatalError on append
tfdiags: Unwrap NonFatalError on append

This is a variant of diagnosticsAsError that we use to signal to informed
callers that there might just be warnings inside, but we should also do
the right thing if a caller just appends it to an existing diagnostics
without checking first.

pryorda/terraform,gechr/terraform,iljaskevic/terraform,pradeepbhadani/terraform,gechr/terraform,tpounds/terraform,iljaskevic/terraform,pryorda/terraform,mattray/terraform,youhong316/terraform,brandontosch/terraform,rnaveiras/terraform,gechr/terraform,nlamirault/terraform,youhong316/terraform,heimweh/terraform,apparentlymart/terraform,hashicorp/terraform,luis-silva/terraform,luis-silva/terraform,hashicorp/terraform,iljaskevic/terraform,drebes/terraform,rnaveiras/terraform,pdecat/terraform,heimweh/terraform,tommynsong/terraform,VladRassokhin/terraform,luis-silva/terraform,brandontosch/terraform,youhong316/terraform,tommynsong/terraform,mattray/terraform,ewbankkit/terraform,apparentlymart/terraform,pdecat/terraform,youhong316/terraform,heimweh/terraform,pradeepbhadani/terraform,drebes/terraform,tpounds/terraform,apparentlymart/terraform,ewbankkit/terraform,apparentlymart/terraform,luis-silva/terraform,ewbankkit/terraform,drebes/terraform,ewbankkit/terraform,tpounds/terraform,iljaskevic/terraform,nlamirault/terraform,nlamirault/terraform,mattray/terraform,pdecat/terraform,drebes/terraform,hashicorp/terraform,heimweh/terraform,pradeepbhadani/terraform,brandontosch/terraform,pryorda/terraform,VladRassokhin/terraform,tommynsong/terraform,VladRassokhin/terraform,apparentlymart/terraform,mattray/terraform,pdecat/terraform,tpounds/terraform,brandontosch/terraform,VladRassokhin/terraform,rnaveiras/terraform,heimweh/terraform,brandontosch/terraform