Repository: typescript-eslint/tslint-to-eslint-config (ts)

Issue:
Unclear error message when tslint command is not found
### 🐛 Bug Report

-   `tslint-to-eslint-config` version: 2.0.0
-   ESLint version: N/A
-   Node version: N/A

#### Actual Behavior

```
❌ Could not start tslint-to-eslint: ❌
  Command failed: tslint --print-config "./tslint.json"
/bin/sh: tslint: command not found
```

#### Expected Behavior

```
❌ Could not start tslint-to-eslint: ❌
    The 'tslint' command was not found. Either install it globally or add it as a devDependency of this repository.
```

#### Reproduction

Per @kumaresan-subramani's info in #823:

```
git clone https://github.com/kumaresan-subramani/ESlint-tester
cd ESlint-tester
npx tslint-to-eslint-config
```

I'm guessing this hasn't been reported until now because most projects that use TSLint and would want to convert to ESLint already have it as a dependency in their `package.json`.

I already have a fix for it from just testing this out locally, so I'll send a PR 😄 . 

Pull request description:
Added explicit error for TSLint not being found
## PR Checklist

-   [x] Addresses an existing issue: fixes #836
-   [x] That issue was marked as [`status: accepting prs`](https://github.com/typescript-eslint/tslint-to-eslint-config/labels/status%3A%20accepting%20prs)

## Overview

Continues #827 but adding another hardcoded error for the _"command not found"_ case.

Patch:
diff --git a/src/input/findTSLintConfiguration.ts b/src/input/findTSLintConfiguration.ts
index 24e3d52..b39240f 100644
--- a/src/input/findTSLintConfiguration.ts
+++ b/src/input/findTSLintConfiguration.ts
@@ -19,6 +19,13 @@ export type FindTSLintConfigurationDependencies = {
 };
 
 const knownErrors = [
+    [
+        "command not found",
+        () =>
+            new Error(
+                "The 'tslint' command was not found. Either install it globally or add it as a devDependency of this repository.",
+            ),
+    ],
     [
         "unknown option `--print-config",
         () => new Error("TSLint v5.18 required. Please update your version."),


Test patch:
diff --git a/src/input/findTslintConfiguration.test.ts b/src/input/findTslintConfiguration.test.ts
index cf0e9b4..fd69586 100644
--- a/src/input/findTslintConfiguration.test.ts
+++ b/src/input/findTslintConfiguration.test.ts
@@ -49,6 +49,25 @@ describe("findTSLintConfiguration", () => {
         );
     });
 
+    it("replaces an error with a v5.18 request when the tslint command is not found", async () => {
+        // Arrange
+        const stderr = "/bin/sh tslint: command not found";
+        const dependencies = createStubDependencies({
+            exec: createStubThrowingExec({ stderr }),
+        });
+
+        // Act
+        const result = await findTSLintConfiguration(dependencies, undefined);
+
+        // Assert
+        expect(result).toEqual(
+            expect.objectContaining({
+                message:
+                    "The 'tslint' command was not found. Either install it globally or add it as a devDependency of this repository.",
+            }),
+        );
+    });
+
     it("replaces an error with a v5.18 request when the --print-config option is unsupported", async () => {
         // Arrange
         const stderr = "unknown option `--print-config";