No helpful error when you call eta over a source file with other extension than .hs

[jneira]

With eta the error is:

>eta qs.eta
eta: panic! (the 'impossible' happened)
  (Eta version 0.0.6):
        Parsing of archive structure failed:
Cannot locate end of central directory
in "path/to/qs.eta"

but ghc outputs a somewhat better error:

>stack exec ghc qs.eta
C:/path/to/stack/i386-windows/ghc-7.10.3/mingw/bin/ld.exe:qs.eta: file format not recognized; treating as linker script
C:/path/to/stack/AppData/Local/Programs/stack/i386-windows/ghc-7.10.3/mingw/bin/ld.exe:qs.eta:1: syntax error
collect2.exe: error: ld returned 1 exit status

(reported originally in #405)