Give Tab warning only once per file

Currently, when compiling a tabbed file, GHC spits out a huge number of warnings:

Main.hs:1:1: Warning: Tab character

[..]

Main.hs:89:1: Warning: Tab character

Main.hs:90:1: Warning: Tab character

Main.hs:91:1: Warning: Tab character

Main.hs:92:1: Warning: Tab character

Main.hs:99:4: Warning: Tab character

Main.hs:100:4: Warning: Tab character

Main.hs:101:4: Warning: Tab character

This is annoying and not very nice to our users.

I suggest to replace it by

This file contains tabs (e.g. line n column m); please use spaces instead

or (to be nice to tools that parse this output)

Main.hs:1:1: Tab found here, and in n further locations. Please use spaces instead.

(This is clearly a low priority request, but also possibly some low-hanging fruit for new contributors.)

Trac metadata

Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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