Return a better error when optional deps are not found

[abmusse]

• Node.js version: 12.19.0
• itoolkit version: latest master
• IBM i version: N/A
• XMLSERVICE version: N/A

Describe the bug

In #306, we added guards around requiring optional idb and odbc dependencies. If these dependencies are not installed and an unknowing user attempts to use the transports, the returned error messages are not really clear.

TypeError: Cannot read property 'dbconn' of null
    at Connection.idbCall [as transportCall] (~/nodejs-itoolkit/lib/transports/idbTransport.js:48:24)
    at Connection.run (~/nodejs-itoolkit/lib/Connection.js:187:10)
    at Object.<anonymous> (~/nodejs-itoolkit/example.js:13:12)

TypeError: Cannot read property 'connect' of null
    at Connection.odbcCall [as transportCall] (~/nodejs-itoolkit/lib/transports/odbcTransport.js:67:8)
    at Connection.run (~/nodejs-itoolkit/lib/Connection.js:187:10)
    at Object.<anonymous> (~/nodejs-itoolkit/example.js:13:12)

To Reproduce

Run the following snippet with transport set to idb or odbc on system without those modules installed. An easy example is using idb transport on your laptop given that idb-connector is only valid when run on IBM i systems.

const { Connection, CommandCall } = require('itoolkit');
const { parseString } = require('xml2js');

const connection = new Connection({
  transport: 'idb',
  transportOptions: { username: 'myuser', password: 'mypassword' },
});

const command = new CommandCall({ type: 'cl', command: 'RTVJOBA USRLIBL(?) SYSLIBL(?)' });

connection.add(command);

connection.run((error, xmlOutput) => {
  if (error) {
    throw error;
  }
  parseString(xmlOutput, (parseError, result) => {
    if (parseError) {
      throw parseError;
    }
    console.log(JSON.stringify(result));
  });
});

Expected behavior

A more descriptive error message like the following:

Error: idb-connector module was not found, the idb transport is only valid IBM i systems
    at ~/Connection.js:195:26
    at Connection.idbCall [as transportCall] (~/nodejs-itoolkit/lib/transports/idbTransport.js:31:5)
    at Connection.run (~/nodejs-itoolkit/lib/Connection.js:187:10)
    at Object.<anonymous> (~/nodejs-itoolkit/example.js:13:12)

Error: odbc module was not found, ensure odbc is installed properly.
    at ~/nodejs-itoolkit/lib/Connection.js:195:26
    at Connection.odbcCall [as transportCall] (~/nodejs-itoolkit/lib/transports/odbcTransport.js:31:5)
    at Connection.run (~/nodejs-itoolkit/lib/Connection.js:187:10)
    at Object.<ano
nymous> (~/nodejs-itoolkit/example.js:13:12)

[github-actions]

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