Improve the error message for no matching bootclasspath (#296)

Closes #296

COPYBARA_INTEGRATE_REVIEW=#296 from fmeum:patch-3 6579d51
PiperOrigin-RevId: 763898415
Change-Id: I3a4a07d682e1e1b450b433931f3366bbe3051e3a

Author: fmeum

toolchains/bootclasspath.bzl

@@ -165,10 +165,12 @@ def _bootclasspath_impl(ctx):
             tool_prefix = "tool_" if is_exec else ""
             fail("""
 No Java runtime found to extract the bootclasspath from for --{tool_prefix}java_language_version={language_version} and --{tool_prefix}java_runtime_version={runtime_version}.
-You can:
+The bootclasspath provides the particular version of the Java standard library to compile against.
+You can do one of the following:
 
-    * register a Java runtime with name "{bootstrap_runtime_version}" to provide the bootclasspath or
-    * set --java_language_version to the Java version of an available runtime.
+    * register a Java runtime with name "{bootstrap_runtime_version}" to provide the bootclasspath
+    * set --java_language_version=N for an available runtime "{runtime_version}_N"
+    * remove the suffix from your runtime's name to use it as the bootclasspath
 
 Rerun with --toolchain_resolution_debug='@bazel_tools//tools/jdk:bootstrap_runtime_toolchain_type' to see more details about toolchain resolution.
 """.format(