Detect and report vcvarsall.bat errors (#28)

If the parameters passed to the script are incorrect -- for example,
architecture is set to something the script does not understand --
then the script will print an error message *and* exit successfully
without doing anything useful.

Detect the error messages, forward them to the user, and fail the
action. Hopefully, the information from the script will be enough
to pinpoint the source of the issue.

For example, if the action is run

    with:
      arch: Win32

then the output will be

    Found with vswhere: C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Auxiliary\Build\vcvarsall.bat
    Error: [ERROR:vcvarsall.bat] Invalid argument found : Win32
    Error: Could not setup Developer Command Prompt: invalid parameters

which is close enough.

index.js

@@ -104,6 +104,24 @@ function main() {
     core.debug(`Running: ${command}`)
     const environment = child_process.execSync(command, {shell: "cmd"}).toString().split('\r\n')
 
+    // If vsvars.bat is given an incorrect command line, it will print out
+    // an error and *still* exit successfully. Parse out errors from output
+    // which don't look like environment variables, and fail if appropriate.
+    var failed = false
+    for (let line of environment) {
+        if (line.match(/^\[ERROR.*\]/)) {
+            failed = true
+            // Don't print this particular line which will be confusing in output.
+            if (line.match(/Error in script usage. The correct usage is:$/)) {
+                continue
+            }
+            core.error(line)
+        }
+    }
+    if (failed) {
+        throw new Error('invalid parameters')
+    }
+
     for (let string of environment) {
         const [name, value] = string.split('=')
         for (let pattern of InterestingVariables) {