Better message and exit code when merged directory cannot be written

[kevmoo]

with Dart SDK 2.0.0-dev.47.0

• build_runner 0.8.2+2
• build_web_compilers 0.3.6

Get exit code 1 and output

    [INFO] Initializing inputs
    [INFO] Reading cached asset graph...
    [INFO] Reading cached asset graph completed, took 304ms

    [INFO] Checking for updates since last build...
    [INFO] Checking for updates since last build completed, took 667ms

    [INFO] Running build...
    [INFO] Running build completed, took 17ms

    [INFO] Caching finalized dependency graph...
    [INFO] Caching finalized dependency graph completed, took 118ms

    [WARNING] Skipped creation of the merged output directory.
[e] [SEVERE] Failed after 148ms
[e] Failed to create merged output directories.

Not helpful.

Suggestion:

• Exit code 73 - "cannot create output file"
• Something about "target directory XYZ already exists. Choose a different directory or delete the contents of that directory"