Add better error handling if cfn-lint is not installed (#10)

* Add better error handling if cfn-lint is not installed

Author: fatbasstard

README.md

@@ -12,11 +12,13 @@ For example if there is an image subfolder under your extension project workspac
 
 ## Requirements
 
-Requires cfn-lint to be installed.  `pip install cfn-lint`
+Requires cfn-lint to be installed: `pip install cfn-lint`
+
+More information about cfn-lint can be found [here](https://github.com/awslabs/cfn-python-lint)
 
 ## Extension Settings
 
 * `cfnLint.path`: path to the cfn-lint command
 * `cfnLint.appendRules`: Array of paths containing additional Rules
 * `cfnLint.ignoreRules`: Array of Rule Ids to be ignored
-* `cfnLint.overrideSpecPath`: Path to an Specification overrule file
+* `cfnLint.overrideSpecPath`: Path to an Specification overrule file

server/src/server.ts

@@ -165,6 +165,21 @@ function validateCloudFormationFile(document: TextDocument): void {
 		let start = 0;
 		let end = Number.MAX_VALUE;
 
+		child.on('error', function(err) {
+			let errorMessage = `Unable to start cfn-lint (${err}). Is cfn-lint installed correctly?`;
+			connection.console.log(errorMessage);
+			let lineNumber = 0;
+			let diagnostic: Diagnostic = {
+				range: {
+					start: { line: lineNumber, character: start },
+					end: { line: lineNumber, character: end }
+				},
+				severity: DiagnosticSeverity.Error,
+				message: errorMessage
+			};
+			diagnostics.push(diagnostic);
+		});
+
 		child.stderr.on("data", (data: Buffer) => {
 			let err = data.toString();
 			connection.console.log(err);