Error message if upload is not possible

[francescospissu]

Motivation

If the user presses the upload button and no board is selected, a message should appear warning what went wrong while uploading. Since verify takes place before upload, the same message that is given when the user tries to compile a sketch without selecting a board should be shown. On the other hand, this message is not very clear, so it should be updated.

Change description

Communicate to user when upload is impossible due to no board selection.
Make error message from compiling without board selected more user friendly.

Other information

Closes #845.
Closes #62.

This fix depends on the changes made here: arduino/arduino-cli#1848. Without it it is not possible to verify that #62 is resolved.

Reviewer checklist

• PR addresses a single concern.
• The PR has no duplicates (please search among the Pull Requests before creating one)
• PR title and description are properly filled.
• Docs have been added / updated (for bug fixes / features)

[kittaakos]

The code looks good to me.

I am not familiar with the board user fields logic, maybe we should change

if (this.boardRequiresUserFields && !this.cachedUserFields.has(key)) {

to

if (this.boardRequiresUserFields && key && !this.cachedUserFields.has(key)) {

[ubidefeo]

Works as expected

[per1234]

Thanks Francesco!