Improve the select boot failure help.

[jsirois]

This fixes the error messages displayed when a boot command could not be
found by both differentiating the cases where this can occur with their
own specialized messages and improving the guidance in general.

As part of these messaging fixes, support for hiding commands is added
by omitting descriptions for those named commands you wish to hide from
the boot failure help screen.

Fixes #143
Fixes #144

[jsirois]

For the motivating user-confusion case from scie-pants, I applied this diff to its science lift manifest:

diff --git a/package/scie-pants.toml b/package/scie-pants.toml
index 6c9cabc..9b02909 100644
--- a/package/scie-pants.toml
+++ b/package/scie-pants.toml
@@ -53,7 +53,7 @@ exe = "{scie-pants.bin}"
 # Run Pants
 [[lift.commands]]
 name = "pants"
-description = "Runs a hermetic Pants installation."
+#description = "Runs a hermetic Pants installation."
 exe = "{scie.bindings.install:PANTS_CLIENT_EXE}"
 args = [
     "{scie.bindings.configure:PANTS_SHA_FIND_LINKS}",
@@ -69,7 +69,7 @@ _PANTS_SERVER_EXE = "{scie.bindings.install:PANTS_SERVER_EXE}"
 # Run Pants Debug Mode
 [[lift.commands]]
 name = "pants-debug"
-description = "Runs a hermetic Pants installation with a debug server for debugging Pants code."
+#description = "Runs a hermetic Pants installation with a debug server for debugging Pants code."
 exe = "{scie.bindings.install:VIRTUAL_ENV}/bin/pants"
 args = [
     "-c",

And now, for the users specific case you get:

$ export SCIE_BOOT="update pants"
$ ./dist/better-help/pants
Error: `SCIE_BOOT=update pants` was found in the environment but "update pants" does not correspond to any scie-pants commands.

Isolates your Pants from the elements.

Please select from the following boot commands:

<default> (when SCIE_BOOT is not set in the environment)  Detects the current Pants installation and launches it.
bootstrap-tools                                           Introspection tools for the Pants bootstrap process.
update                                                    Update scie-pants.

You can select a boot command by setting the SCIE_BOOT environment variable.

[huonw]

Nice