Improve parse error message on indentation mistake

Consider the following code fragment:

                   (ExecGhc, args) -> execCompiler "" args
                    -- NOTE: this won't currently work for GHCJS, because it doesn't have
                    -- a runghcjs binary. It probably will someday, though.
                    (ExecRunGhc, args) ->

The last line is indented by one space but it is hard to notice esp when we looking at this along with a screenful of code and there are many comments between the two lines. When this code is compiled ghc throws the following error:

Main.hs:745:40: error: parse error on input ‘->’

I look at the error and then the code and wonder what is wrong with this code, there is nothing noticeably wrong. If ghc can provide some context I can easily make out where the problem is. ghc can perhaps tell me how the current expression is being parsed e.g. (ExecRunGhc, args) -> is being considered as a continuation of the expression on the previous line.

I have run into this many times. The first time it was really frustrating.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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