Detect java not being in PATH at all, document java PATH requirement

docs/CONTRIBUTING/Dependencies.md

@@ -118,6 +118,8 @@ export ANDROID_NDK=<path_to_ndk_root>
 export JAVA_HOME=<path_to_jdk_root>
 ```
 
+You must also make sure that you have the `java` from `JAVA_HOME` in your `PATH`, as some Android build commands run java directly without respecting `JAVA_HOME`.
+
 Otherwise, below are steps to acquire the tools for each platform. These steps download specifically the versions listed above, other versions may work but are not guaranteed.
 
 ### Android Dependencies on Windows

renderdoccmd/CMakeLists.txt

@@ -92,9 +92,13 @@ if(ANDROID)
     set(SAVE_JAVA_HOME $ENV{JAVA_HOME})
 
     set(ENV{JAVA_HOME} "")
-    find_package(Java REQUIRED)
+    find_package(Java)
     set(ENV{JAVA_HOME} ${SAVE_JAVA_HOME})
 
+    if(NOT ${Java_FOUND})
+        message(FATAL_ERROR "Building Android requires the 'java' program in your PATH. It must be at least Java 8 (1.8)")
+    endif()
+
     if(${Java_VERSION} VERSION_LESS 1.8)
         message(FATAL_ERROR "Building Android requires the 'java' program in your PATH to be at least Java 8 (1.8)")
     endif()