Implement new abstraction for file paths

Once integrated with the rest of the installer, this should fix many directory traversal and path encoding issues

src/main/kotlin/link/infra/packwiz/installer/request/RequestExceptions.kt

@@ -0,0 +1,70 @@
+package link.infra.packwiz.installer.request
+
+import okhttp3.Response
+import okio.IOException
+
+sealed class RequestException: Exception {
+	constructor(message: String, cause: Throwable) : super(message, cause)
+	constructor(message: String) : super(message)
+
+	/**
+	 * Internal errors that should not be shown to the user when the code is correct
+	 */
+	sealed class Internal: RequestException {
+		constructor(message: String, cause: Throwable) : super(message, cause)
+		constructor(message: String) : super(message)
+
+		class UnsinkableBase: Internal("Base associated with this path is not a SinkableBase")
+
+		sealed class HTTP: Internal {
+			constructor(message: String, cause: Throwable) : super(message, cause)
+			constructor(message: String) : super(message)
+
+			class NoResponseBody: HTTP("HTTP response in onResponse must have a response body")
+			class RequestFailed(cause: IOException): HTTP("HTTP request failed; may have been cancelled", cause)
+			class IllegalState(cause: IllegalStateException): HTTP("Internal fatal HTTP request error", cause)
+		}
+	}
+
+	/**
+	 * Errors indicating that the request is malformed
+	 */
+	sealed class Validation: RequestException {
+		constructor(message: String, cause: Throwable) : super(message, cause)
+		constructor(message: String) : super(message)
+
+		// TODO: move out of RequestException?
+		class PathContainsNUL(path: String): Validation("Invalid path; contains NUL bytes: ${path.replace("\u0000", "")}")
+		class PathContainsVolumeLetter(path: String): Validation("Invalid path; contains volume letter: $path")
+	}
+
+	/**
+	 * Errors relating to the response from the server
+	 */
+	sealed class Response: RequestException {
+		constructor(message: String, cause: Throwable) : super(message, cause)
+		constructor(message: String) : super(message)
+
+		// TODO: fancier way of displaying this?
+		sealed class HTTP: Response {
+			val response: okhttp3.Response
+
+			constructor(response: okhttp3.Response, message: String, cause: Throwable) : super(message, cause) {
+				this.response = response
+			}
+			constructor(response: okhttp3.Response, message: String) : super(message) {
+				this.response = response
+			}
+
+			class ErrorCode(res: okhttp3.Response): HTTP(res, "Non-successful error code from HTTP request: ${res.code}")
+		}
+
+		sealed class File: RequestException {
+			constructor(message: String, cause: Throwable) : super(message, cause)
+			constructor(message: String) : super(message)
+
+			class FileNotFound(file: String): File("File path not found: $file")
+			class Other(cause: Throwable): File("Failed to read file", cause)
+		}
+	}
+}