v.pref: fix `v build file.v`

cmd/v/help/default.txt

@@ -34,7 +34,7 @@ V supports the following commands:
    list              List all installed modules.
    outdated          Show installed modules that need updates.
 * Others:
-   build             Build a V code in the provided path (the default, so you can skip the word `build`).
+   build             Build V code in the provided path (the default, so you can skip the word `build`).
    translate         Translate C code to V (coming soon in 0.3).
    tracev            Produce a tracing version of the v compiler.
                      Use `tracev yourfile.v` when the compiler panics.

cmd/v/v.v

@@ -87,7 +87,7 @@ fn main() {
 		}
 		else {}
 	}
-	if command in ['run', 'build-module'] || command.ends_with('.v') || os.exists(command) {
+	if command in ['run', 'build', 'build-module'] || command.ends_with('.v') || os.exists(command) {
 		// println('command')
 		// println(prefs.path)
 		builder.compile(command, prefs)

vlib/v/pref/pref.v

@@ -310,6 +310,13 @@ pub fn parse_args(args []string) (&Preferences, string) {
 	}
 	if command.ends_with('.v') || os.exists(command) {
 		res.path = command
+	} else if command == 'build' {
+		if command_pos + 2 != args.len {
+			eprintln('`v build` requires exactly one argument - either a single .v file, or a single folder/ containing several .v files')
+			exit(1)
+		}
+		res.path = args[command_pos + 1]
+		must_exist(res.path)
 	} else if command == 'run' {
 		res.is_run = true
 		if command_pos + 2 > args.len {
@@ -318,6 +325,7 @@ pub fn parse_args(args []string) (&Preferences, string) {
 		}
 		res.path = args[command_pos + 1]
 		res.run_args = args[command_pos + 2..]
+		must_exist(res.path)
 	}
 	if command == 'build-module' {
 		res.build_mode = .build_module
@@ -327,6 +335,13 @@ pub fn parse_args(args []string) (&Preferences, string) {
 	return res, command
 }
 
+fn must_exist(path string) {
+	if !os.exists(path) {
+		eprintln('v expects that `$path` exists, but it does not')
+		exit(1)
+	}        
+}
+
 pub fn backend_from_string(s string) ?Backend {
 	match s {
 		'c' { return .c }