fix: throw new error object so stack traces are valid

If the thrown error was not created where it's thrown, the stack
trace does not reflect where the error was thrown from making it
harder to debug problems.

This PR removes the cached error object and instead creates a new
one when required.

src/index.js

@@ -13,8 +13,6 @@ exports.isEncoded = isEncoded
 exports.names = Object.freeze(Object.keys(constants.names))
 exports.codes = Object.freeze(Object.keys(constants.codes))
 
-const errNotSupported = new Error('Unsupported encoding')
-
 /**
  * Create a new buffer with the multibase varint+code.
  *
@@ -120,7 +118,7 @@ function getBase (nameOrCode) {
   } else if (constants.codes[nameOrCode]) {
     base = constants.codes[nameOrCode]
   } else {
-    throw errNotSupported
+    throw new Error('Unsupported encoding')
   }
 
   if (!base.isImplemented()) {