v.util: improve code related to diff tool specified via environment, add check if the diff tool exists

[ttytm]

The PR works on the util.diff module's code related to the VDIFF_TOOL environment variable.
It cleans up to help with maintainability and slightly extends by adding an error that should improve usability.

Regarding the changes:
At the beginning of the function we already checked if there is an env_difftool and return early. How env_difftool was used after this in the function, was quasi dead code.

v/vlib/v/util/diff/diff.v

Lines 11 to 13 in 4dcc926

	if env_difftool != '' {
	return '${env_difftool} ${env_diffopts}'
	}

Then, instead of just returning the strings specified in the environment string it is also ensured here that specified command is executable. If not an error is returned to the user. E.g. when it isn't installed or if a typo or something.

[spytheman]

Excellent work.