Add "Error:" prefix for compile-time error messages

We propose adding the prefix Error: to compile-time error messages produced by GHC.

An example of a warning message (see Warning: prefix):

➜ cat test.hs
main = putStrLn "Hello!"
➜ ghc --make -fforce-recomp -Wall ./test.hs
[1 of 1] Compiling Main             ( test.hs, test.o )

test.hs:1:1: Warning:
    Top-level binding with no type signature: main :: IO ()
Linking test ...

An example of a current error message:

➜ cat test2.hs
main = foo
➜ ghc --make -fforce-recomp -Wall ./test2.hs
[1 of 1] Compiling Main             ( test2.hs, test2.o )

test2.hs:1:8: Not in scope: ‘foo’

The proposal is to change the error message to become:

➜ cat test2.hs
main = foo
➜ ghc --make -fforce-recomp -Wall ./test2.hs
[1 of 1] Compiling Main             ( test2.hs, test2.o )

test2.hs:1:8: Error: Not in scope: ‘foo’

This change affects only the error messages produced by GHC when compiling. It does not change runtime error reporting, e.g. with the error function.

Motivation

We wish to make compile-time error messages easier for humans and computers to identify. For example, when running a long parallel build, it can be easy to miss an error message (see #9219 for an example). With a known string such as Error:, we can use search functionality in a terminal window or text editor (when viewing the build log) or use a command-line tool such as grep (when streaming and filtering the build log) to quickly find errors.

As a secondary motivation, adding Error: for error messages brings a nice symmetry to the current use of Warning: for warning messages.

Disadvantages

The output of many regression tests will need to be updated for the new error message format.

Additional consideration

@hvr [ticket:10021#comment:95657 suggested] that during work on this bug we could make GHC output more consistent with other compilers (GCC and clang) by using lowercase error: and warning: prefixes. This might be potentially useful for existing tools that work on top of compiler output for finding errors/warning, and GHC's output will become "more valid" for them.