cmd/evm: change error msg output to stderr

[terasum]

use the ctx.Writer and ctx.ErrWriter as output witer,

before:

$> ./evm compile test.sol 2>/dev/null
test.sol:0 syntax error: unexpected hello, expected end of line
test.sol:0 syntax error: unexpected EOF, expected new line

after:

$> ./evm compile test.sol 2>/dev/null

[terasum]

@karalabe I'm sorry fixed an insignificant code, but can you take a look, please?

[fjl]

This is a good change, but please fix the linter errors: https://travis-ci.org/ethereum/go-ethereum/jobs/399649306#L502

[terasum]

@fjl Thanks for your review, I have fixed the linter error, would you like take a look again?

[karalabe]

LGTM

[karalabe]

Unfortunately this PR actually broke the evm utility. ctx.App.ErrWriter is usually nil, which causes evm to panic. As such, I don't really see the benefit of this rewrite any more, will revert.