Add error.isMaxBuffer

[ehmicky]

This adds error.isMaxBuffer: boolean. This is true when the subprocess failed due to the maxBuffer option.

Figuring out whether the subprocess failed due to the maxBuffer option is not completely straightforward otherwise:

• execa() fails with MaxBufferError instances (which Execa does not export)
• execaSync() fails with error.code ENOBUFS. However that code is also used for other errors by Node.js.

Also, this PR improves the error message when maxBuffer is hit. Previously, it just showed maxBuffer exceeded. Now, the error message is clearer. Also, it indicates:

• Which stream was too big: stdout, stderr, stdio[3], etc.
• The maxBuffer value
• The unit: bytes, characters, lines or objects

[ehmicky]

Fixed merge conflict.