Large amount of errors if not fully installed

[ianchanning]

• OS: Fedora 28
• Vim: v8.1
• Default browser: Firefox v61.0.2

N.B. This is purely a 'newbie' mistake after first installing the plugin, but having not run the cargo build in the plugin directory.

After installing the plugin (via Vundle) this plugin generates errors on any kind of keypress with markdown files:

Error detected while processing function <SNR>120_sendBuffer:
line    6:
E906: Not an open channel

I also get when switching buffers:

Error detected while processing function <SNR>157_chdir:
line    7:
E906: Not an open channel
Error detected while processing function <SNR>157_sendBuffer:
line    6:
E906: Not an open channel
Press ENTER or type command to continue

The startup message is very helpful:

Could not execute markdown composer: try `cargo build --release --no-default-features --features json-rpc` in the plugin directory

However would it be possible to do some kind of error handling to prevent this large number of errors before the plugin is fully working?