Added specific permission getting

ref #6

src/errors/PermissionErrors.ts

@@ -0,0 +1,25 @@
+import { IsString } from 'class-validator';
+import { NotAcceptableError, NotFoundError } from 'routing-controllers';
+
+/**
+ * Error to throw when a permission couldn't be found.
+ */
+export class PermissionNotFoundError extends NotFoundError {
+	@IsString()
+	name = "PermissionNotFoundError"
+
+	@IsString()
+	message = "Permission not found!"
+}
+
+/**
+ * Error to throw when two permission' ids don't match.
+ * Usually occurs when a user tries to change a permission's id.
+ */
+export class PermissionIdsNotMatchingError extends NotAcceptableError {
+	@IsString()
+	name = "PermissionIdsNotMatchingError"
+
+	@IsString()
+	message = "The id's don't match!! \n And if you wanted to change a permission's id: This isn't allowed"
+}

src/errors/PrincipalsErrors.ts

@@ -0,0 +1,13 @@
+import { IsString } from 'class-validator';
+import { NotFoundError } from 'routing-controllers';
+
+/**
+ * Error to throw when a user couldn't be found.
+ */
+export class PrincipalNotFoundError extends NotFoundError {
+	@IsString()
+	name = "PrincipalNotFoundError"
+
+	@IsString()
+	message = "Principal not found!"
+}