Fix install.js error message

[NikitaEgorov]

Please change

if (err) {
	log.error(err.stack);
	return;
}

to

if (err) {
	log.error(err);
	return;
}

[sindresorhus]

Why? .stack includes the error message too:

(new Error('foo')).stack
"Error: foo
    at <anonymous>:1:2"

[NikitaEgorov]

In build process it is look like:

/build/node_modules/optipng-bin # node lib/install.js
  ⚠ spawn /build/node_modules/optipng-bin/vendor/optipng ENOENT
  ⚠ optipng pre-build test failed
  ℹ compiling from source
  ✖ Error: ./configure --with-system-zlib --prefix="/build/node_modules/optipng-bin/vendor" --bindir="/build/node_modules/optipng-bin/vendor" && make install
Command failed: ./configure --with-system-zlib --prefix="/build/node_modules/optipng-bin/vendor" --bindir="/build/node_modules/optipng-bin/vendor"

    at ChildProcess.exithandler (child_process.js:206:12)
    at emitTwo (events.js:106:13)
    at ChildProcess.emit (events.js:191:7)
    at maybeClose (internal/child_process.js:877:16)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:226:5)