Make run and docs commands fail with more accurate instructions

[stefanv]

We should not propage errors, such as FileNotFound, through to the user.

For run, this means not telling the user to file spin bugs. For other commands, just try and catch as many known exceptions and give helpful feedback.

xref: #260 #262

[stefanv]

Since we launch subprocess for run, that one should not be a problem. The issue that prompted this was actually a custom command that raised internally, because it did not check for the presence of the binary it called. I clarified the error message shown by spin in that case in #266